Pharmacological Characterization of the Zebrafish (Danio Rerio) Histamine H1 Receptor Reveals the Involvement of the Second Extracellular Loop in the Binding of Histamine.

The zebrafish (Danio rerio) histamine H1 receptor gene (zfH1R) was cloned in 2007 and reported to be involved in fish locomotion. Yet, no detailed characterization of its pharmacology and signaling properties have so far been reported. In this study, we pharmacologically characterized the zfH1R expressed in HEK-293T cells by means of [3H]-mepyramine binding and G protein-signaling assays. The zfH1R [dissociation constant (KD), 0.7 nM] displayed similar affinity for the antagonist [3H]-mepyramine as the human histamine H1 receptor (hH1R) (KD, 1.5 nM), whereas the affinity for histamine is 100-fold higher than for the human H1R. The zfH1R couples to Gαq/11 proteins and activates several reporter genes, i.e., NFAT, NFÏ°B, CRE, VEGF, COX-2, SRE, and AP-1, and zfH1R-mediated signaling is prevented by the Gαq/11 inhibitor YM-254890 and the antagonist mepyramine. Molecular modeling of the zfH1R and human H1R shows that the binding pockets are identical, implying that variations along the ligand binding pathway could underly the differences in histamine affinity instead. Targeting differentially charged residues in extracellular loop 2 (ECL2) using site-directed mutagenesis revealed that Arg21045x55 is most likely involved in the binding process of histamine in zfH1R. This study aids the understanding of the pharmacological differences between H1R orthologs and the role of ECL2 in histamine binding and provides fundamental information for the understanding of the histaminergic system in the zebrafish. SIGNIFICANCE STATEMENT: The use of the zebrafish as in vivo models in neuroscience is growing exponentially, which asks for detailed characterization of the aminergic neurotransmitter systems in this model. This study is the first to pharmacologically characterize the zebrafish histamine H1 receptor after expression in HEK-293T cells. The results show a high pharmacological and functional resemblance with the human ortholog but also reveal interesting structural differences and unveils an important role of the second extracellular loop in histamine binding.

Effects of membrane transport activity and cell metabolism on the unbound drug concentrations in the skeletal muscle and liver of drugs: A microdialysis study in rats.

The unbound concentrations of 14 commercial drugs, including five non-efflux/uptake transporter substrates-Class I, five efflux transporter substrates-class II and four influx transporter substrates-Class III, were simultaneously measured in rat liver, muscle, and blood via microanalysis. Kpuu,liver and Kpuu,muscle were calculated to evaluate the membrane transport activity and cell metabolism on the unbound drug concentrations in the skeletal muscle and liver. For Class I compounds, represented by antipyrine, unbound concentrations among liver, muscle and blood are symmetrically distributed when compound hepatic clearance is low. And when compound hepatic clearance is high, unbound concentrations among liver, muscle and blood are asymmetrically distributed, such as Propranolol. For Class II and III compounds, overall, the unbound concentrations among liver, muscle, and blood are asymmetrically distributed due to a combination of hepatic metabolism and efflux and/or influx transporter activity.

Ca2+ -dependent down-regulation of human histamine H1 receptors in Chinese hamster ovary cells.

Gq/11 protein-coupled human histamine H1 receptors in Chinese hamster ovary cells stimulated with histamine undergo clathrin-dependent endocytosis followed by proteasome/lysosome-mediated down-regulation. In this study, we evaluated the effects of a sustained increase in intracellular Ca2+ concentrations induced by a receptor-bypassed stimulation with ionomycin, a Ca2+ ionophore, on the endocytosis and down-regulation of H1 receptors in Chinese hamster ovary cells. All cellular and cell-surface H1 receptors were detected by the binding of [3 H]mepyramine to intact cells sensitive to the hydrophobic and hydrophilic H1 receptor ligands, mepyramine and pirdonium, respectively. The pretreatment of cells with ionomycin markedly reduced the mepyramine- and pirdonium-sensitive binding sites of [3 H]mepyramine, which were completely abrogated by the deprivation of extracellular Ca2+ and partially by a ubiquitin-activating enzyme inhibitor (UBEI-41), but were not affected by inhibitors of calmodulin (W-7 or calmidazolium) and protein kinase C (chelerythrine or GF109203X). These ionomycin-induced changes were also not affected by inhibitors of receptor endocytosis via clathrin (hypertonic sucrose) and caveolae/lipid rafts (filipin or nystatin) or by inhibitors of lysosomes (E-64, leupeptin, chloroquine, or NH4 Cl), proteasomes (lactacystin or MG-132), and a Ca2+ -dependent non-lysosomal cysteine protease (calpain) (MDL28170). Since H1 receptors were normally detected by confocal immunofluorescence microscopy with an antibody against H1 receptors, even after the ionomycin treatment, H1 receptors appeared to exist in a form to which [3 H]mepyramine was unable to bind. These results suggest that H1 receptors are apparently down-regulated by a sustained increase in intracellular Ca2+ concentrations with no process of endocytosis and lysosomal/proteasomal degradation of receptors.

Pyrilamine-sensitive proton-coupled organic cation (H+/OC) antiporter for brain-specific drug delivery.

Blood-brain barrier (BBB) represents the greatest challenge that hampers therapeutic molecules entering the brain. Here, we described a novel brain-specific delivery strategy targeting to pyrilamine-sensitive H+/OC antiporter to facilitate therapeutic molecules cross the BBB and penetrate into the brain. In this study, four cyclic tertiary amines were selected as the brain-targeting moieties to modify naproxen (NP), a non-steroidal anti-inflammatory drug. The obtained NP conjugates displayed cell uptake efficiencies over 144-fold higher than that of unmodified NP in endothelial cells. The cell uptake process of the conjugates was primarily driven by pyrilamine-sensitive H+/OC antiporter in a pH-dependent, Na+-independent, and membrane potential-independent pathway, which could be further inhibited by pyrilamine, propranolol, and imipramine. Moreover, the NP conjugates showed significantly higher AUC0-t and Cmax in the brain compared with unmodified NP, and significantly higher accumulation than NP in the in situ brain perfusion study. Also, the conjugates showed superior neuroprotective effect in vitro and in vivo. Thus, the chemical modification of therapeutics with a cyclic tertiary amine moiety represents a promising and efficient strategy for brain-specific drug delivery via pyrilamine-sensitive H+/OC antiporter.

Functional expression of nicotine influx transporter in A549 human alveolar epithelial cells.

Nicotine is a potent addictive alkaloid, and is rapidly absorbed through the alveoli of the lung. However, the transport mechanism of nicotine at the human alveolar epithelial barrier has not been investigated in great detail. In the present study, the transport mechanism of nicotine across alveolar epithelium was investigated in vitro using A549 cells, a human adenocarcinoma-derived cell line with an alveolar epithelial cell like phenotype. Nicotine uptake by A549 cells exhibited time-, temperature-, and concentration-dependence with a Km of 50.4 µM. These results suggest that a carrier-mediated transport process is involved in nicotine transport in human alveolar epithelial cells. Nicotine uptake by A549 cells was insensitive to change in extracellular pH. Moreover, nicotine uptake by A549 cells could be inhibited by organic cations such as verapamil and pyrilamine, but not typical substrates of organic cation transporters and ß2-agonist. These results suggest that a novel, not yet molecularly identified, organic cation transporter plays a role in nicotine transport which is unlikely to interact with ß2-agonist transport. This nicotine influx transporter in human alveolar epithelium might have implications for the rapid absorption of nicotine into the systemic circulation.

Involvement of the H+/organic cation antiporter in nicotine transport in rat liver.

Nicotine is an addictive alkaloid in cigarette smoke and is responsible for tobacco dependence. It is important to consider the blood-to-liver transport of nicotine to understand the nicotine elimination from the body because most of the nicotine is converted to inactive metabolites by cytochrome P450 localized in the endoplasmic reticulum of the hepatocytes. In this study, the blood-to-liver transport of nicotine was investigated by means of an in vivo portal vein injection technique in rats, and the in vitro uptake by freshly isolated rat hepatocytes was used to clarify its mechanism. The results obtained showed that the in vivo blood-to-liver transport of [(3)H]nicotine was significantly inhibited by 50 mM nicotine and pyrilamine, suggesting involvement of a carrier-mediated transport process in the blood-to-liver transport of nicotine. The in vitro uptake study using freshly isolated rat hepatocytes showed a time- and concentration-dependent uptake of [(3)H]nicotine with a Km value of 141 µM, and the uptake was increased under alkaline extracellular conditions. In addition, intracellular acidification caused an increase in [(3)H]nicotine uptake, suggesting that the influx transport of nicotine is driven by an oppositely directed H(+) gradient in hepatocytes. Moreover, [(3)H]nicotine uptake was strongly inhibited in the presence of cationic drugs, such as pyrilamine, whereas only weak inhibitory effects were shown by substrates of typical organic cation transporters, such as tetraethylammonium, 1-methyl-4-phenylpyridinium, choline, and l-carnitine. In conclusion, a carrier-mediated system controlling the blood-to-liver transport of nicotine appears to be present on the sinusoidal membrane of hepatocytes. The pattern of inhibition and ion dependence is suggestive of an H(+)/organic cation antiporter-mediated nicotine transport system.

Effects of antihistamines on the function of human α7-nicotinic acetylcholine receptors.

Effects of the histamine H1 receptor (H1R) antagonists (antihistamines), promethazine (PMZ), orphenadrine (ORP), chlorpheniramine (CLP), pyrilamine (PYR), diphenhydramine (DPH), citerizine (CTZ), and triprolidine (TRP) on the functional properties of the cloned α7 subunit of the human nicotinic acetylcholine receptor expressed in Xenopus oocytes were investigated. Antihistamines inhibited the α7-nicotinic acetylcholine receptor in the order PYR>CLP>TRP>PMZ>ORP≥DPH≥CTZ. Among the antihistamines, PYR showed the highest reversible inhibition of acetylcholine (100 µM)-induced responses with IC50 of 6.2 µM. PYR-induced inhibition was independent of the membrane potential and could not be reversed by increasing the concentration of acetylcholine. Specific binding of [¹²5I] α-bungarotoxin, a selective antagonist for α7-nicotinic acetylcholine receptor, was not changed in the presence of PYR suggesting a non-competitive inhibition of nicotinic receptors. In line with functional experiments, docking studies indicated that PYR can potentially bind allosterically with the α7 transmembrane domain. Our results indicate that the H2-H4 receptor antagonists tested in this study (10 µM) showed negligible inhibition of α7-nicotinic acetylcholine receptors. On the other hand, H1 receptor antagonists inhibited the function of human α7-nicotinic acetylcholine receptor, with varying potencies. These results emphasize the importance of α7-nicotinic acetylcholine receptor for future pharmacological/toxicological profiling.

Pyrilamine inhibits nicotine-induced catecholamine secretion.

Function of nicotine, which induces activation of all parts of the body including our brain, has been receiving much attention for a long period of time and also been actively studied by researchers for its pharmacological actions in the central nervous system. The modulation of nicotine concentration and the inhibition of nicotine binding on target receptors in the brain are the key factors for smoking addiction therapy. In previous studies showed that influx of nicotine at the blood-brain barrier was through the pyrilamine-sensitive organic cation transporters. But the direct interacting mechanism of pyrilamine on the nicotine binding target receptors has not yet been clarified. The aim of the present study is to investigate the direct binding mechanisms of a pyrilamine on the nicotinic acetylcholine receptors (nAChRs). We found that pyrilamine shares the same ligand binding pocket of nicotine (NCT) on nAChRs but interacts with more amino acid residues than NCT does. The extended part of pyrilamine interacts with additional residues in the ligand binding pocket of nAChRs which are located nearby the entrance of the binding pocket. The catecholamine (CA) secretion induced by nAChR agonist (NCT') was significantly inhibited by the pyrilamine pretreatment. Real time carbon-fiber amperometry confirmed the inhibition of the NCT'-induced exocytosis by pyrilamine in a single cell level. We also found that pyrilamine inhibited the NCT'-induced [Ca(2+)]i. In contrast, pyrilamine did not affect the increase in calcium induced by high K(+). Overall, these data suggest that pyrilamine directly docks into the ligand binding site of nAChRs and specifically inhibits the nAChR-mediated effects thereby causing inhibition of CA secretion. Therefore, pyrilamine may play an important role to explore new treatments to aid smoking cessation.

Vasopeptidase-activated latent ligands of the histamine receptor-1.

Whether peptidases present in vascular cells can activate prodrugs active on vascular cells has been tested with 2 potential latent ligands of the histamine H1 receptor (H1R). First, a peptide consisting of the antihistamine cetirizine (CTZ) condensed at the N-terminus of ε-aminocaproyl-bradykinin (εACA-BK) was evaluated for an antihistamine activity that could be revealed by degradation of the peptide part of the molecule. CTZ-εACA-BK had a submicromolar affinity for the BK B2 receptor (B2R; IC50 of 590 nM, [(3)H]BK binding competition), but a non-negligible affinity for the human H1 receptor (H1R; IC50 of 11 µM for [(3)H]pyrilamine binding). In the human isolated umbilical vein, a system where both endogenous B2R and H1R mediate strong contractions, CTZ-εACA-BK exerted mild antagonist effects on histamine-induced contraction that were not modified by omapatrilat or by a B2R antagonist that prevents endocytosis of the BK conjugate. Cells expressing recombinant ACE or B2R incubated with CTZ-εACA-BK did not release a competitor of [(3)H]pyrilamine binding to H1Rs. Thus, there is no evidence that CTZ-εACA-BK can release free cetirizine in biological environments. The second prodrug was a blocked agonist, L-alanyl-histamine, potentially activated by aminopeptidase N (APN). This compound did not compete for [(3)H]pyrilamine binding to H1Rs. The human umbilical vein contractility assay responded to L-alanyl-histamine (EC50 54.7 µM), but the APN inhibitor amastatin massively (17-fold) reduced its apparent potency. Amastatin did not influence the potency of histamine as a contractile agent. One of the 2 tested latent H1R ligands, L-alanyl-histamine, supported the feasibility of pro-drug activation by vascular ectopeptidases.

Homologous desensitization of histamine-mediated signal transduction system in C6 glioma cells.

Molecular events involved in the homologous desensitization of histamine-mediated signal transduction system in glioma cells are not well understood. The aim of this study was designed to gain further insight into possible events in the process using the C6 glioma cells. Incubation of histamine caused increases in inositol phosphate (IP1) formation and intracellular free-calcium concentration [Ca2+]i in C6 glioma cells via the activation of a G-protein-coupled phospholipase C (PI-PLC). Histamine also caused an increase in extracellular release of arachidonic acid (AA) and formation of glycerophosphoinositol (GPI). These effects are likely to be mediated through the activation of receptor-coupled phospholipase A2 (PLA2). Pretreatment of C6 cells with histamine, from 0.1 microM to 1 mM concentrations, for 10 to 60 min significantly reduced the histamine-induced IP1 production, [Ca2+]i accumulation, AA release and GPI formation, despite repeated wash of the cells with buffer solution. Staurosporine (10 nM), a protein kinase C (PKC) inhibitor, reversed almost completely IP1 production, or partially for [Ca2+]i, GPI formation and AA release of this homologous desensitization effect of histamine. Pretreatment of C6 cells with phorbol 12-myristate 13-acetate (PMA), a PKC activator, at 0.1 nM to 0.1 microM for 2 to 15 min caused a reduction of histamine-induced IP1 formation and [Ca2+] accumulation, but enhanced histamine-induced AA release and GPI formation. Ten nM staurosporine completely reversed the effect of PMA on histamine-induced IP1 formation and partially on [Ca2+]i accumulation. However, staurosporine potentiated the effect of PMA on histamine-induced AA release and GPI formation, but the effect could be blocked by H7, a calcium-dependent PKC inhibitor. Our results indicate that activation of PKC by histamine in the signal transduction system is involved in the histamine-induced homologous desensitization event. Since PMA pretreatment could not mimic histamine-induced homologues desensitization event in AA release and GPI formation, it is likely due to the dual actions of this protein kinase activator: on calcium independently, and also on calcium dependent via influx of calcium ion through the plasma membrane. The calcium flux effect of PMA is related to the difference between PMA and histamine on the effects of AA release and GPI formation via activation on PLA2. The results of this study provided strong evidence that PKC is involved in this homologous desensitization caused by continuous histamine receptor activation.

Involvement of a novel organic cation transporter in verapamil transport across the inner blood-retinal barrier.

PURPOSE: To clarify the transport and inhibition characteristics involved in verapamil transport across the inner blood-retinal barrier (inner BRB). METHODS: The transport of [(3)H]verapamil across the inner BRB was investigated using retinal uptake index and integration plot analyses in rats. The detailed transport characteristics were studied using TR-iBRB2 cells, a conditionally immortalized rat retinal capillary endothelial cell line that is an in vitro model of the inner BRB. RESULTS: The apparent influx permeability clearance of [(3)H]verapamil was 614 µL/(min·g retina), which is 4.7-fold greater than that of brain. The retinal uptake of [(3)H]verapamil was slightly increased by 3 mM verapamil and 10 mM qunidine and inhibited by 40 mM pyrilamine, supporting the carrier-mediated efflux and influx transport of verapamil across the inner BRB. TR-iBRB2 cells exhibited a concentration-dependent uptake of [(3)H]verapamil with a K (m) of 61.9 µM, and the uptake was inhibited by several cations, such as pyrilamine, exhibiting a different profile from the identified transporters. These transport properties suggest that verapamil transport at the inner BRB takes place via a novel organic cation transporter. CONCLUSIONS: Our findings suggest that a novel organic cation transporter is involved in verapamil transport from the blood to the retina across the inner BRB.

Investigation on signal transduction pathways after H(1) receptor activated by histamine in C6 glioma cells: involvement of phosphatidylinositol and arachidonic acid metabolisms.

BACKGROUND: Information related to histamine-induced cellular responses in C6 glioma cells through second messenger pathways has not been fully studied, especially the involvement of arachidonic acid (AA) metabolism. In addition, specific labeled ligand binding to histamine receptor sites still needs to be clarified. METHODS: Labeled mepyramine ligand was used to study its binding sites; [(3)H] inositol was used to detect inositol 4-phosphate (IP(1)) formation, and fura-2/AM was used to detect intracellular free calcium ion ([Ca(2+)]i) level activated by the phosphatidylinositol-phospholipase C (PI-PLC) pathway. Also, labeled AA was used to detect the metabolism of AA and its metabolites release via the activation of phospholipase A2 in the presence of histamine. RESULTS: C6 glioma cells incubated with histamine in the presence of 10 mM LiCl for 60 minutes induced an increase of IP(1) and glycerophosphoric-inositol (GPI) accumulation. In addition, histamine caused an increase of extracellular AA with its metabolite release, eliciting a transient and sustained increase of free [Ca(2+)]i. The sustained increase of [Ca(2+)]i was almost or completely blocked by La(3+) and excess ethylene diamine tetraacetic acid. The calcium ion influx associated with the sustained phase required the presence of histamine on the receptor sites, and could be blocked by a H(1) antagonist, chlorpheniramine. CONCLUSION: C6 glioma cells possess histamine H(1) receptors that have affinity towards [(3)H]mepyramine binding, and are coupled to PI-PLC to generate inositol phosphates and to increase [Ca(2+)]i, and they are coupled to phospholipase A2 (PLA2) to generate GPI and AA with its metabolite release. The transient increase in [Ca(2+)]i can be attributed to Ca(2+) release from intracellular stores, whereas the sustained increase in [Ca(2+)]i is due to influx of extracellular calcium ions. The sustained increase in [Ca(2+)]i plays a role in the activation of histamine receptor-coupled PLA2.

Influence of the N-terminus and the E2-loop onto the binding kinetics of the antagonist mepyramine and the partial agonist phenoprodifen to H(1)R.

Numerous competitive radioligand binding studies revealed significant differences between human and guinea pig histamine H(1)-receptors (hH(1)R and gpH(1)R), e.g. for the partial H(1)R agonist phenoprodifen. But until now, there are only few studies with regard to binding kinetics at H(1)R. Previous studies from our group revealed an influence of the exchange of N-terminus and E2-loop between hH(1)R and gpH(1)R onto affinity of phenoprodifen to H(1)R (Strasser A, Wittmann HJ, Seifert R, J Pharmacol Exp Ther 326:783-791, 2008). The aim of this study was, therefore, to examine the impact of the N-terminus and the E2-loop on binding kinetics of the H(1)R. The wild type hH(1)R and gpH(1)R and the chimeric h(gpE2)H(1)R (E2-loogp from guinea pig) and h(gpNgpE2)H(1)R (N-terminus and E2-loop from guinea pig) were co-expressed with regulator of G-protein signaling protein RGS4 in Sf9 insect cells and kinetic binding studies were performed using the antagonist [(3)H]mepyramine as radioligand. The rate constants for association and dissociation were, in dependence of the ligand, different between hH(1)R and gpH(1)R. Furthermore, the rate constants for association at h(gpNgpE2)H(1)R were significantly different compared to hH(1)R and gpH(1)R. Molecular dynamic simulation studies detected different interactions of amino acid side chains on the extracellular surface of the receptor. Based on these findings, the influence of extracellular surface onto binding kinetics and binding affinity can be explained. Thus, the extracellular surface of G protein-coupled receptors for biogenic amines, exhibits influence onto kinetics of ligand binding, onto ligand recognition and ligand guiding into the binding pocket.

Lipopolysaccharide induces H1 receptor expression and enhances histamine responsiveness in human coronary artery endothelial cells.

Summary Histamine is a well-recognized modulator of vascular inflammation. We have shown that histamine, acting via H1 receptors (H1R), synergizes lipopolysaccharide (LPS)-induced production of prostaglandin I(2) (PGI(2)), PGE(2) and interleukin-6 (IL-6) by endothelial cells. The synergy between histamine and LPS was partly attributed to histamine -induced expression of Toll-like receptor 4 (TLR4). In this study, we examined whether LPS stimulates the H1R expression in human coronary artery endothelial cells (HCAEC) with resultant enhancement of histamine responsiveness. Incubation of HCAEC with LPS (10-1000 ng/ml) resulted in two-fold to fourfold increases in H1R mRNA expression in a time-dependent and concentration-dependent fashion. In contrast, LPS treatment did not affect H2R mRNA expression. The LPS-induced H1R mRNA expression peaked by 4 hr after LPS treatment and remained elevated above the basal level for 20-24 hr. Flow cytometric and Western blot analyses revealed increased expression of H1R protein in LPS-treated cells. The specific binding of [(3)H]pyrilamine to H1R in membrane proteins from LPS-treated HCAEC was threefold higher than the untreated cells. The LPS-induced H1R expression was mediated through TLR4 as gene silencing by TLR4-siRNA and treatment with a TLR4 antagonist inhibited the LPS effect. When HCAEC were pre-treated with LPS for 24 hr, washed and challenged with histamine, 17-, 10- and 15-fold increases in PGI(2), PGE(2) and IL-6 production, respectively, were noted. Histamine-induced enhancement of the synthesis of PGI(2), PGE(2) and IL-6 by LPS-primed HCAEC was completely blocked by an H1R antagonist. The results demonstrate that LPS, through TLR4 activation, up-regulates the expression and function of H1R and amplifies histamine-induced inflammatory responses in HCAEC.

Activation of brain histaminergic neurotransmission: a mechanism for cognitive effects of memantine in Alzheimer's disease.

We previously reported that some N-methyl-D-aspartate (NMDA)-receptor antagonists enhanced histamine neuron activity in rodents. Here, we have investigated the effects of memantine, an NMDA-receptor antagonist used for the treatment of Alzheimer's disease, on histaminergic neurotransmission. In vitro, memantine antagonized native NMDA receptors with a micromolar potency but had no effect at recombinant human histamine receptors. In vivo, a single administration of memantine increased histamine neuron activity, as shown by the 60% increase of tele-methylhistamine (t-MeHA) levels observed in the brain of mice. This increase occurred with an ED(50) of 0.3 ± 0.1 mg/kg, similar to that found on inhibition of ex vivo [(3)H]dizocilpine maleate (MK-801) binding (1.8 ± 1.3 mg/kg). Two days after pretreatment of mice with memantine at 5 mg/kg twice daily for 5 days, t-MeHA levels were enhanced by 50 ± 7% (p < 0.001), indicating a long-lasting activation of histamine neurons. Quantitative polymerase chain reaction analysis was used to explore genes involved in this persistent effect. H(3) receptor mRNAs were strongly increased, but the density of H(3) receptor binding sites was increased solely in hypothalamus (by 141 ± 24%). Up-regulations of brain-derived neurotrophic factor and NMDA-receptor 1 subunit mRNAs were also found but were restricted to hippocampus. mRNA expression of α7-nicotinic receptors remained unchanged in any region. Considering the well established cognitive effects of histamine neurons, the increase in brain t-MeHA levels after single or repeated administration of therapeutic doses of memantine suggests that the drug exerts its beneficial effects on cognitive deficits of Alzheimer's disease, at least partly, by activating histamine neurons.

Kujin suppresses histamine signaling at the transcriptional level in toluene 2,4-diisocyanate-sensitized rats.

Kujin, the dried root of Sophorae flavescensis, has been used in Chinese folklore medicine against allergy. Evaluation of its anti-allergic potential as well as its mechanism of action has rarely been established. We investigated the effect of Kujin on toluene-2,4-diisocyanate (TDI)-induced allergic behavior and related histamine signaling including mRNA levels of histamine H(1) receptor (H1R) and histidine decarboxylase (HDC), H1R and HDC activities, and histamine content in rat nasal mucosa. We also investigated the effect of Kujin on the mRNA levels of helper T cell type 2 (Th2)-cytokine genes closely related to histamine signaling. TDI provocation caused acute allergic symptoms accompanied with up-regulations of H1R and HDC mRNAs and increases in HDC activity, histamine content, and [(3)H]mepyramine binding activity in the nasal mucosa, all of which were significantly suppressed by pretreatment with Kujin for 3 weeks. Kujin also suppressed the TDI-induced IL-4 and IL-5 mRNA elevations. These data suggest that oral administration of Kujin showed anti-allergic activity through suppression of histamine signaling by the inhibition of TDI-induced H1R and HDC mRNA elevations followed by decrease in H1R, HDC protein level, and histamine content in the nasal mucosa of TDI-sensitized rats. Suppression of Th2-cytokine signaling by Kujin also suggests that it could affect the histamine-cytokine network.

Pyrilamine in the horse: detection and pharmacokinetics of pyrilamine and its major urinary metabolite O-desmethylpyrilamine.

Pyrilamine is an antihistamine used in human and veterinary medicine. As antihistamines produce central nervous system effects in horses, pyrilamine has the potential to affect the performance of racehorses. In the present study, O-desmethylpyrilamine (O-DMP) was observed to be the predominant equine urinary metabolite of pyrilamine. After intravenous (i.v.) administration of pyrilamine (300 mg/horse), serum pyrilamine concentrations declined from about 280 ng/mL at 5 min postdose to about 2.5 ng/mL at 8 h postdose. After oral administration of pyrilamine (300 mg/horse), serum concentrations peaked at about 33 ng/mL at 30 min, falling to <2 ng/mL at 8 h postdose. Pyrilamine was not detected in serum samples at 24 h postdosing by either route. After i.v. injection of pyrilamine (300 mg/horse) O-DMP was recovered at a level of about 20 microg/mL at 2 h postdose thereafter declining to about 2 ng/mL at 168 h postdose. After oral administration, the O-DMP recovery peaked at about 12 microg/mL at 8 h postdose and declined to <2 ng/mL at 168 h postdose. These results show that pyrilamine is poorly bioavailable orally (18%), and can be detected by sensitive enzyme-linked immunosorbent assay tests in urine for up to 1 week after a single administration. Care should be taken as the data suggest that the withdrawal time for pyrilamine after repeated oral administrations is likely to be at least 1 week or longer.

Pharmacological profile of histaprodifens at four recombinant histamine H1 receptor species isoforms.

There are differences in the pharmacological properties of phenylhistamines and histaprodifens between guinea pig histamine H(1) receptor (gpH(1)R) and human histamine H(1) receptor (hH(1)R). The aim of this study was to analyze species differences in more detail, focusing on histaprodifen derivatives and including the bovine histamine H(1) receptor (bH(1)R) and rat histamine H(1) receptor (rH(1)R). H(1)R species isoforms were coexpressed with the regulator of G protein signaling RGS4 in Sf9 insect cells. We performed [(3)H]mepyramine binding assays and steady-state GTPase assays. For a novel class of histaprodifens, the chiral histaprodifens, unique species differences between hH(1)R, bH(1)R, rH(1)R, and gpH(1)R were observed. The chiral histaprodifens 8R and 8S were both partial agonists at gpH(1)R, but only 8R was a partial agonist at the other H(1)R species isoforms. An additional phenyl group in chiral histaprodifens 10R and 10S, respectively, resulted in a switch from agonism at gpH(1)Rto antagonism at hH(1)R, bH(1)R, and rH(1)R. In general, histaprodifens showed the order of potency hH(1)R < bH(1)R < rH(1)R < gpH(1)R. An active-state model of gpH(1)R was generated with molecular dynamics simulations. Dimeric histaprodifen was docked into the binding pocket of gpH(1)R. Hydrogen bonds and electrostatic interactions were detected between dimeric histaprodifen and Asp-116, Ser-120, Lys-187, Glu-190, and Tyr-432. We conclude the following: 1) chiral histaprodifens interact differentially with H(1)R species isoforms; 2) gpH(1)R and rH(1)R, on one hand, and hH(1)R and bH(1)R, on the other hand, resemble each other structurally and pharmacologically; and 3) histaprodifens interact with H(1)R at multiple sites.