Inhibition of Astrocytic Histamine N-Methyltransferase as a Possible Target for the Treatment of Alzheimer's Disease.

Alzheimer's disease (AD) represents the principal cause of dementia among the elderly. Great efforts have been established to understand the physiopathology of AD. Changes in neurotransmitter systems in patients with AD, including cholinergic, GABAergic, serotoninergic, noradrenergic, and histaminergic changes have been reported. Interestingly, changes in the histaminergic system have been related to cognitive impairment in AD patients. The principal pathological changes in the brains of AD patients, related to the histaminergic system, are neurofibrillary degeneration of the tuberomammillary nucleus, the main source of histamine in the brain, low histamine levels, and altered signaling of its receptors. The increase of histamine levels can be achieved by inhibiting its degrading enzyme, histamine N-methyltransferase (HNMT), a cytoplasmatic enzyme located in astrocytes. Thus, increasing histamine levels could be employed in AD patients as co-therapy due to their effects on cognitive functions, neuroplasticity, neuronal survival, neurogenesis, and the degradation of amyloid beta (Aß) peptides. In this sense, the evaluation of the impact of HNMT inhibitors on animal models of AD would be interesting, consequently highlighting its relevance.

Metoprine, a histamine N-methyltransferase inhibitor, attenuates methamphetamine-induced hyperlocomotion via activation of histaminergic neurotransmission in mice.

Metoprine increases the content of histamine in brain by inhibiting histamine N-methyltransferase (HMT), a centrally acting histamine degrading enzyme. We present data demonstrating that pretreatment with metoprine attenuates the hyperlocomotive effects of METH in mice using a multi-configuration behavior apparatus designed to monitor four behavioral outcomes [horizontal locomotion, appetitive behavior (food access), and food and water intake]. Metoprine pretreatment itself induced hyperlocomotion in mice challenged with saline during the large part of light phase. The trend was also observed during the following dark phase. This is the first report that metoprine has a long-lasting locomotor stimulating property. Similarly, in a tail suspension test, a single injection of metoprine significantly reduced total time of immobility in mice, consistent with the idea that metoprine possesses motor stimulating properties. Metoprine pretreatment did not affect other aspects of behavior. Metoprine did not affect the appetitive and drinking behavior while exerted an effect on stereotypy. No stereotyped behavior was observed in mice pretreated with vehicle followed by METH, while stereotyped sniffing was observed in mice pretreated with metoprine followed by METH. The metoprine pretreatment attenuated METH-induced hyperlocomotion during the first 2 h of light phase, suggesting that metoprine-induced locomotor stimulating property might be different from that of METH. The hypothalamic content of histamine (but not its brain metabolite) was increased after metoprine or METH administration. Both METH and metoprine reduced dopamine and histamine turnover in the striatum and the nucleus accumbens and the hypothalamus, respectively, and there is a significant metoprine pretreatment x METH challenge interaction in the histamine turnover. It is likely that metoprine may attenuate METH-induced hyperlocomotion via activation of histaminergic neurotransmission. Metoprine also might induce a long-lasting locomotor stimulating effect via a putative mechanism different from that whereby METH induces the locomotor stimulating effect.

Histamine N-Methyltransferase in the Brain.

Brain histamine is a neurotransmitter and regulates diverse physiological functions. Previous studies have shown the involvement of histamine depletion in several neurological disorders, indicating the importance of drug development targeting the brain histamine system. Histamine N-methyltransferase (HNMT) is a histamine-metabolising enzyme expressed in the brain. Although pharmacological studies using HNMT inhibitors have been conducted to reveal the direct involvement of HNMT in brain functions, HNMT inhibitors with high specificity and sufficient blood⁻brain barrier permeability have not been available until now. Recently, we have phenotyped Hnmt-deficient mice to elucidate the importance of HNMT in the central nervous system. Hnmt disruption resulted in a robust increase in brain histamine concentration, demonstrating the essential role of HNMT in the brain histamine system. Clinical studies have suggested that single nucleotide polymorphisms of the human HNMT gene are associated with several brain disorders such as Parkinson's disease and attention deficit hyperactivity disorder. Postmortem studies also have indicated that HNMT expression is altered in human brain diseases. These findings emphasise that an increase in brain histamine levels by novel HNMT inhibitors could contribute to the improvement of brain disorders.

Optically active iodohelicene derivatives exhibit histamine N-methyl transferase inhibitory activity.

Optically active helicene derivatives inhibit the activity on histamine N-methyl transferase (HNMT). Specifically, methyl (P)-1,12-dimethylbenzo[c]phenanthrene-8-carboxylate with 6-iodo and 5-trifluoromethanesulfonyloxy groups inhibits HNMT activity on the µM order of IC50. Chirality is important, and (M)-isomers exhibits substantially reduced activity. The 6-iodo group is also essential, which suggests the involvement of halogen bonds in protein binding. Substituents on the sulfonate moiety also affect the inhibitory activity.

Cleansing the world of the germ of laziness: hygiene, sanitation, and the Javanese population in Suriname / Limpando o mundo do germe da preguiça: higiene, saneamento e população javanesa no Suriname

In 1915 the Rockefeller Foundation took its hookworm eradication campaign to Suriname, but was soon disappointed because of opposition from its main target group: the Javanese. Moreover, authorities and planters objected to the construction of latrines because of the costs and their belief that the Javanese were “unhygienic”. In describing the labor migration from Java to Suriname, I show that this “lack of hygiene” was closely related to the system’s organization. I argue that uncleanliness was the consequence of harmful socio-economic and ecological conditions. Secondly I suggest that even though the Foundation did not manage to cleanse Suriname of hookworm, its educational efforts, its emphasis on prevention, and its training of local health workers probably had more impact than Rockefeller officials thought.

Em 1915, a Fundação Rockefeller levou sua campanha de erradicação da ancilostomíase ao Suriname, logo sofrendo a oposição de seu principal alvo, os javaneses. Autoridades e proprietários rurais também reagiram à instalação de latrinas devido aos custos implicados e à crença de que os javaneses eram “anti-higiênicos”. Ao descrever a migração de trabalhadores de Java para o Suriname, mostro que a “falta de higiene” ligava-se à organização do sistema. Argumento que a sujeira era consequência de condições ecológicas e socioeconômicas danosas. Sugiro ainda que, embora a Fundação não tenha livrado o Suriname da anciolostomíase, seus esforços educacionais, sua ênfase na prevenção e o treinamento de profissionais de saúde locais tiveram maior impacto do que o imaginado pelos funcionários da agência norte-americana.

Alterations in the histaminergic system in the substantia nigra and striatum of Parkinson's patients: a postmortem study.

Earlier studies showed neuronal histamine production in the hypothalamic tuberomamillary nucleus to be unchanged in Parkinson's disease (PD), whereas the histamine levels and innervation in the substantia nigra (SN) increased. In the present study we used quantitative polymerase chain reaction (qPCR) to assess the changes in the histaminergic system in the SN, caudate nucleus (CN), and putamen (PU) in 7 PD patients and 7 controls. The messenger RNA (mRNA) expression of the histamine receptor-3 (H(3)R), which was localized immunocytochemically in the large pigmented neurons, was significantly decreased in the SN in PD, while histamine receptor-4 (H(4)R)-mRNA expression showed a significant increase in caudate nucleus and PU. In addition, significantly increased mRNA levels of histamine methyltransferase (HMT), a key enzyme involved in histamine metabolism, were found in the SN and in the PU in PD. Moreover, in the SN, the histamine methyltransferase-mRNA showed a strong negative correlation with PD disease duration. Our observations imply the presence of local changes in the histaminergic system that may contribute to PD pathology, and may thus provide a rationale for possible novel therapeutic strategies.

Histamine facilitates consolidation of fear extinction.

Non-reinforced retrieval induces memory extinction, a phenomenon characterized by a decrease in the intensity of the learned response. This attribute has been used to develop extinction-based therapies to treat anxiety and post-traumatic stress disorders. Histamine modulates memory and anxiety but its role on fear extinction has not yet been evaluated. Therefore, using male Wistar rats, we determined the effect of the intra-hippocampal administration of different histaminergic agents on the extinction of step-down inhibitory avoidance (IA), a form of aversive learning. We found that intra-CA1 infusion of histamine immediately after non-reinforced retrieval facilitated consolidation of IA extinction in a dose-dependent manner. This facilitation was mimicked by the histamine N-methyltransferase inhibitor SKF91488 and the H2 receptor agonist dimaprit, reversed by the H2 receptor antagonist ranitidine, and unaffected by the H1 antagonist pyrilamine, the H3 antagonist thioperamide and the antagonist at the NMDA receptor (NMDAR) polyamine-binding site ifenprodil. Neither the H1 agonist 2-2-pyridylethylamine nor the NMDAR polyamine-binding site agonist spermidine affected the consolidation of extinction while the H3 receptor agonist imetit hampered it. Extinction induced the phosphorylation of ERK1 in dorsal CA1 while intra-CA1 infusion of the MEK inhibitor U0126 blocked extinction of the avoidance response. The extinction-induced phosphorylation of ERK1 was enhanced by histamine and dimaprit and blocked by ranitidine administered to dorsal CA1 after non-reinforced retrieval. Taken together, our data indicate that the hippocampal histaminergic system modulates the consolidation of fear extinction through a mechanism involving the H2-dependent activation of ERK signalling.

Identification of inhibitors of Plasmodium falciparum phosphoethanolamine methyltransferase using an enzyme-coupled transmethylation assay.

BACKGROUND: The phosphoethanolamine methyltransferase, PfPMT, of the human malaria parasite Plasmodium falciparum, a member of a newly identified family of phosphoethanolamine methyltransferases (PMT) found solely in some protozoa, nematodes, frogs, and plants, is involved in the synthesis of the major membrane phospholipid, phosphatidylcholine. PMT enzymes catalyze a three-step S-adenosylmethionine-dependent methylation of the nitrogen atom of phosphoethanolamine to form phosphocholine. In P. falciparum, this activity is a limiting step in the pathway of synthesis of phosphatidylcholine from serine and plays an important role in the development, replication and survival of the parasite within human red blood cells. RESULTS: We have employed an enzyme-coupled methylation assay to screen for potential inhibitors of PfPMT. In addition to hexadecyltrimethylammonium, previously known to inhibit PfPMT, two compounds dodecyltrimethylammonium and amodiaquine were also found to inhibit PfPMT activity in vitro. Interestingly, PfPMT activity was not inhibited by the amodiaquine analog, chloroquine, or other aminoquinolines, amino alcohols, or histamine methyltransferase inhibitors. Using yeast as a surrogate system we found that unlike wild-type cells, yeast mutants that rely on PfPMT for survival were sensitive to amodiaquine, and their phosphatidylcholine biosynthesis was inhibited by this compound. Furthermore NMR titration studies to characterize the interaction between amoidaquine and PfPMT demonstrated a specific and concentration dependent binding of the compound to the enzyme. CONCLUSION: The identification of amodiaquine as an inhibitor of PfPMT in vitro and in yeast, and the biophysical evidence for the specific interaction of the compound with the enzyme will set the stage for the development of analogs of this drug that specifically inhibit this enzyme and possibly other PMTs.

Influence of magnetic resonance contrast media on the activity of histamine inactivating enzymes.

RATIONALE AND OBJECTIVES: The observation that the intravenous application of gadolinium-based contrast media can lead to nephrogenic systemic fibrosis (NSF) has raised interest in their interactions with pathways. In this context, histamine is a focus because of its stimulating effect on fibrogenesis. In humans, histamine can be inactivated either by diamine oxidase (DAO) or by histamine N-methyltransferase (HMT), and numerous drugs are known to inhibit these enzymes. Therefore, it was the aim of this study to investigate whether magnetic resonance imaging contrast agents have an inhibitory effect on the enzymatic activities of DAO and HMT. MATERIALS AND METHODS: Seven gadolinium-based (gadoterate meglumine, gadoteridol, gadobutrol, gadobenate dimeglumine, gadopentetate dimeglumine, gadoxetate disodium, gadodiamide) and one manganese-containing (mangafodipir) contrast agents were tested in vitro. Following the preincubation of purified DAO and HMT with 0.1 to 10 mmol/L of the respective contrast medium, enzyme activities were determined using radiometric microassays. Enzyme activities measured in the absence of contrast agents and after preincubation with specific inhibitors of DAO and HMT, respectively, served as controls. RESULTS: The gadolinium-containing and manganese-containing contrast media tested did not show significant inhibition of the activities of DAO and HMT. No significant difference was observed between ionic and nonionic or between cyclic and linear gadolinium compounds. Preincubation of the enzymes with specific inhibitors led to complete inhibition of the respective enzymatic activity. CONCLUSION: The gadolinium-containing and manganese-based magnetic resonance imaging contrast media tested did not exhibit significant inhibition of histamine-inactivating enzymes at physiologically relevant concentrations.

Bioelimination of histamine in epithelia of the porcine proximal colon of pigs.

OBJECTIVE: The study aimed to gain insight into how intestinal histamine N-methyltransferase (HMT) and diamine oxidase (DAO) could contribute to bioelimination of histamine. MATERIAL AND METHODS: Mucosal-to-serosal (ms) and serosal-to-mucosal (sm) fluxes of histamine, choline or 5-hydroxytryptamine were measured in isolated colonic epithelia of pigs (Ussing chambers). RESULTS: Radioactively (hist-rad) and HPLC-measured histamine fluxes were higher in sm vs. ms direction at 100 microM histamine. Choline (3-3000 microM) and 5-HT (20 microM) fluxes only tended to be higher in sm vs. ms direction. Secretion of hist-rad was abolished by serosal 1-methyl-4-phenylpyridinium (MPP). Histamine fluxes accounted for <25 % of hist-rad fluxes, but this percentage increased after blocking HMT (100 microM amodiaquin) or DAO (100 microM aminoguanidine). 1-Methylhistamine (1-MH) appeared exclusively in the serosal medium. 1-MH appearance decreased after addition of amodiaquin or after addition of N-ethylmaleimide (1 mM NEM). Blockage of vesicular trafficking by NEM enhanced histamine catabolism, which could be reversed by aminoguanidine. DAO was detected in punctuate structures in the basal parts of colonocytes by immunohistochemistry. CONCLUSIONS: A basolateral organic cation transporter facilitates histamine secretion into the intestinal lumen and delivers histamine to catabolism by HMT and/or vesicular DAO. Histamine is partially released back into the blood after initial biotransformation to 1-MH.

Histamine metabolism influences blood vessel branching in zebrafish reg6 mutants.

BACKGROUND: Vascular branching morphogenesis is responsible for the extension of blood vessels into growing tissues, a process crucial for organogenesis. However, the genetic mechanism for vessel branching is largely unknown. Zebrafish reg6 is a temperature-sensitive mutation exhibiting defects in blood vessel branching which results in the formation of swollen vessel lumina during capillary plexus formation. RESULTS: We performed a screening for chemical suppressors of reg6 and identified SKF91488, an inhibitor of histamine methyltransferase (HMT), that can rescue the reg6 vessel branching defects in a dose-dependent manner. Inhibition of HMT by SKF91488 presumably blocks histamine degradation, thus causing histamine accumulation. Consistent with this idea, we found that a high level of histamine also showed significant suppression of reg6 vessel phenotypes. Interestingly, when reg6 adults that had already developed swollen vessel lumina in regenerating fins were treated with histamine or SKF91488, either treatment significantly reduced the number of swollen vessels within 12 h, suggesting a rapid and constant influence of histamine on blood vessel branching. Furthermore, the expression of HMT was significantly elevated in reg6 regenerating fins. Conversely, lowering histamine by administering urocanic acid, a histidine decarboxylase inhibitor, enhanced the reg6 phenotypes. Finally, we identified that the transcription factor, egr-1 (early growth response factor 1), was closely associated with the reg6 phenotype and chemical treatments. CONCLUSION: Taken together, our results suggest that blood vessel branching is influenced by histamine metabolism, possibly through regulating the expression of the egr-1 transcription factor.

Histaminergic modulation of acoustically induced running behavior in rats.

Explosive running is a reliable initial component of audiogenic seizures (AS) induced by acoustic stimulation in genetically prone rodents. This profound locomotor activation is usually considered as a convulsive manifestation of AS although some studies attribute running to a panic-like response. Increase in central histamine activity has been shown to suppress clonic and tonic seizures. The present study examined the involvement of histaminergic mechanisms in the expression of running component of AS. Metoprine, an inhibitor of histamine-N-methyltransferase, was used to increase brain histamine level. Running was induced 4 and 24 h after intraperitoneal injection of metoprine or vehicle in rats of different strains. A brief sound stimulation elicited running followed by clonic-tonic convulsions in Krushinsky-Molodkina (KM) rats or running alone in AS-prone Wistar and WAG/Rij rats. In KM rats, metoprine exerted opposite effects on the main phases of AS. It increased the duration of running and decreased the duration and severity of clonic-tonic convulsions. In Wistar rats, metoprine produced a remarkable aggravation of running leading to its 2- to 3-fold prolongation. In WAG/Rij rats with mixed seizures (absence and audiogenic), the drug caused either aggravation or suppression of running behavior. These results suggest specific role for histaminergic system in the expression of behavioral components of AS. Suppressive role of histamine in clonic-tonic seizures is associated with facilitation of running suggesting specific effects of histamine on brainstem neuronal networks underlying these phases of AS. Possible roles of histaminergic mechanisms in seizure, motor and aversive aspects of sound-induced running are discussed.

Effect of amodiaquine, a histamine N-methyltransferase inhibitor, on, Propionibacterium acnes and lipopolysaccharide-induced hepatitis in mice.

We examined whether treatment with amodiaquine, a potent inhibitor of histamine N-methyltransferase protects mice from Propionibacterium acnes (P. acnes)-primed and lipopolysaccharide (LPS)-induced hepatitis. The subcutaneous injection of amodiaquine (2 and 5 mg/kg) significantly increased the histamine levels in the liver in comparison to saline treated mice. Pretreatment with amodiaquine also improved the survival rate of the hepatitis mice, and this improvement was partially associated with the decrease in serum levels of aspartate aminotransferase (AST) and alanine aminotransferase (ALT). Amodiaquine partially suppressed increases of tumor necrosis factor (TNF)-alpha in the serum and TNF-alpha mRNA expression in the liver, whereas the expression of interleukin (IL)-18, interferon (IFN)-gamma and IL-12 in the liver was not changed by amodiaquine treatment. In conclusion, the present findings suggested that the elevation of endogenous histamine by amodiaquine may thus play a protective role through the regulation of TNF-alpha production in endotoxin-induced hepatic injury mice.

Multiple enzyme inhibitions by histamine H3 receptor antagonists as potential procognitive agents.

Novel highly affine histamine H3 receptor ligands with additional inhibitory effects on the main histamine metabolizing enzyme in the brain, N-methyltransferase, chemically show structural elements of the acetylcholinesterase inhibitor tacrine. H3 receptor antagonism, inhibition of metabolisation of neuronal histamine as well as inhibition of hydrolysis of acetylcholine are each one believed to improve reduced cognitive functions, which is useful for symptomatic treatment of Alzheimer's disease. Some of the new compounds proved in a slightly modified colorimetric Ellmann's assay to be potent inhibitors of acetylcholinesterase and of butyrylcholinesterase which is another catalytic enzyme hydrolysing acetylcholine. Some compounds with (sub)nanomolar activities on the histamine-related targets are also active in the nanomolar concentration range on both cholinesterase targets being 5- to 40-times more potent than tacrine. Preliminary structure-activity relationships could already be drawn from the small number of compounds. The compounds acting as hybrid drugs simultaneously on four different targets to enhance cognitive functions via different pathways are promising lead structures for a new approach in the treatment of Alzheimer's disease.

The protective effect of H2-receptor activation against the duration of myocardial hypoxia/reoxygenation-induced ventricular fibrillation in sensitized guinea-pig hearts.

Patients with high serum immunoglobulin E levels were reported to be protected against sudden death during acute myocardial infarction. The protection mechanism might be attributed to the facilitation of histamine release from sensitized mast cells; however, this remains to be clarified. In this study, we examined the influence of sensitization on ventricular fibrillation (VF) induced by myocardial hypoxia/reoxygenation (H/R). Guinea pigs were actively sensitized by subcutaneous injection of ovalbumin in Bordetella pertussis vaccine. Hearts isolated from non-sensitized and sensitized guinea pigs were subjected to 30-min hypoxia / 30-min reoxygenation using a Langendorff apparatus. The amount of histamine released in the sensitized guinea-pig hearts was elevated, and the duration of VF was found to be reduced. The treatment with a histamine H2-receptor antagonist inhibited the reduction of VF duration. Treatment of the non-sensitized hearts with the histamine H2-receptor agonist resulted in the decrease of VF duration to the same level as that in the sensitized hearts. In conclusion, these results suggest that the risk of sudden death during myocardial H/R may be attenuated in the sensitized hearts and that histamine H2-receptor activation due to the released histamine may be involved in the protective effect.

Structural basis for inhibition of histamine N-methyltransferase by diverse drugs.

In mammals, histamine action is terminated through metabolic inactivation by histamine N-methyltransferase (HNMT) and diamine oxidase. In addition to three well-studied pharmacological functions, smooth muscle contraction, increased vascular permeability, and stimulation of gastric acid secretion, histamine plays important roles in neurotransmission, immunomodulation, and regulation of cell proliferation. The histamine receptor H1 antagonist diphenhydramine, the antimalarial drug amodiaquine, the antifolate drug metoprine, and the anticholinesterase drug tacrine (an early drug for Alzheimer's disease) are surprisingly all potent HNMT inhibitors, having inhibition constants in the range of 10-100nM. We have determined the structural mode of interaction of these four inhibitors with HNMT. Despite their structural diversity, they all occupy the histamine-binding site, thus blocking access to the enzyme's active site. Near the N terminus of HNMT, several aromatic residues (Phe9, Tyr15, and Phe19) adopt different rotamer conformations or become disordered in the enzyme-inhibitor complexes, accommodating the diverse, rigid hydrophobic groups of the inhibitors. The maximized shape complementarity between the protein aromatic side-chains and aromatic ring(s) of the inhibitors are responsible for the tight binding of these varied inhibitors.

Actions of tacrine and galanthamine on histamine-N-methyltransferase.

Histamine-synthesizing neurons in the brain may play an important role in cognition, and a histaminergic deficit has been found in Alzheimer's disease (AD). The AD medication tacrine was previously shown to inhibit some forms of rodent histamine-N-methyltransferase (HNMT), but the effects of AD drugs have not been investigated on human HNMT activity. Presently, the effects of tacrine and galanthamine (another AD medication) were studied on the activity of several forms of human and rat HNMT. Tacrine (0.01-10 microM) inhibited both human and rat HNMT activity in a concentration-dependent manner, but was less potent on both human embryonic kidney and recombinant human brain HNMT than on rat kidney HNMT (IC50 values were 0.46 and 0.70 microM vs. 0.29 microM, respectively). Galanthamine (up to 10 microM) did not influence the activity of rat kidney or human HNMT. Tacrine, but not galanthamine, may achieve brain levels sufficient to influence histamine metabolism in some patients treated for AD.

Role of histaminergic neurons in development of epileptic seizures in EL mice.

The EL mouse is an animal model for hereditary temporal lobe epilepsy. When the mice receive weekly vestibular stimulation, e.g., 30 "tosses", 10-15 cm vertically, they start to convulse after 1-2 weeks. The aim of this study was to evaluate the role of the histaminergic neurons in the regulation of seizure development in the EL mice. The obtained results indicated that administration of either histidine, a substrate for histamine synthesis, or metoprine (2,4-diamino-5-(3,4-dichlorophnyl)-6-methyl-pyrimidine), an inhibitor of histamine N-methyltransferase (HNMT), retarded the onset of seizure episodes in the mice. The co-administration of histidine and metoprine caused a more marked delay in it. The histamine levels in the brain significantly increased in response to any of these treatments. The intraperitoneal injection of diphenhydramine, a H1-antagonist accelerated the initiation of seizure episodes in the mice, whereas thioperamide, a H3-antagonist caused a delay in the response. There were significant increases in the brain histamine levels upon injection of any of these drugs with concomitant rises in the activity of the histidine decarboxylase (HDC). These results, taken together, suggest that the histaminergic neurons play crucial roles in the development of seizures in the EL mice. They inhibit convulsion in a H1-dependent fashion, while the neurons enhance it in a H3-receptor-mediated way.

Search for histamine H(3) receptor ligands with combined inhibitory potency at histamine N-methyltransferase: omega-piperidinoalkanamine derivatives.

In an effort to design new hybrid compounds with dual properties, i.e. binding affinity at histamine H(3) receptors and inhibitory potency at the catabolic enzyme histamine N(tau)-methyltransferase (HMT), a novel series of 1-substituted piperidine derivatives was synthesized. This alicyclic heterocycle is structurally linked via aminoalkyl spacers of variable lengths to additional aromatic carbo- or hetero-cycles. These new hybrid drugs were pharmacologically evaluated regarding their binding affinities at recombinant human H(3) receptors, stably expressed in CHO cells, and in a functional assay for their inhibitory potencies at rat kidney HMT. All compounds investigated proved to be H(3) receptor ligands with binding affinities in the micro- to nanomolar concentration range despite significant differences in the type of the aromatic moiety introduced. The most potent compound in this series was the quinoline derivative 20 (K(i) = 5.6 nM). Likewise, all new ligands studied showed impressive HMT inhibitory activities. Here, compounds 5, 10, 14 and 18-20 exhibited submicromolar potencies (IC(50) = 0.061-0.56 microM). The aminomethylated quinoline 19 showed almost the same, well balanced nanomolar activities on both targets. In this study, new hybrid compounds with a dual mode biological action were developed. These pharmacological agents are valuable leads for further development and candidates for treatment of histamine-dependent disorders.

Cardiac and regional haemodynamic effects of histamine N-methyltransferase inhibitor metoprine in haemorrhage-shocked rats.

OBJECTIVE AND DESIGN: The increase in central histamine concentrations after inhibition of histamine N-methyltransferase (HNMT) activity is associated with the reversal of critical haemorrhagic hypotension, therefore the present study examines cardiac and regional haemodynamic effects of HNMT inhibitor metoprine in haemorrhage-shocked rats. MATERIAL: Cardiovascular parameters were measured in 72 and central histamine concentrations in 12 male Wistar rats anaesthetised with ketamine/xylazine. TREATMENT: Metoprine (5, 15 mg/kg) was administered intraperitoneally to normotensive and critically-hypotensive rats with mean arterial pressure (MAP) 20-25 mmHg. Haemorrhage-shocked rats were pre-treated intracerebroventricularly with histamine H(3) receptor agonist R(-)-alpha-methylhistamine (10 microg) or saline. METHODS: MAP, heart rate (HR) and cardiac and regional haemodynamics were monitored within 2 h after treatment, or to death if it occurred earlier. Histamine concentrations were measured using enzyme immunoassay. ANOVA followed by Neuman-Keules test, and Fisher's exact test were used to compare the results. RESULTS: Bleeding resulted in an extreme decrease in cardiac index (CI), an increase in total peripheral resistance index (TPRI) and the death of control animals within 30 min. Metoprine induced increases in MAP and HR which were significantly higher in hypotensive than in normotensive animals. The resuscitating effect of metoprine (15 mg/kg) was associated with a rise in CI, a decrease in TPRI, and a 100% survival at 2 h. TPRI changes resulted from decreased renal, hindquarters and mesenteric vascular resistance. R(-)-alpha-methylhistamine inhibited metoprine-induced increases in endogenous histamine concentrations in the cerebral cortex (0.89 +/- 0.12 vs. 1.25 +/- 0.29 nmol/g of wet tissue; P < 0.05), hypothalamus (4.37 +/- 0.42 vs. 5.74 +/- 0.47 nmol/g of wet tissue; P < 0.01) and medulla oblongata (0.39 +/- 0.07 vs. 0.65 +/- 0.28 nmol/g of wet tissue; P < 0.05), diminished haemodynamic effects and decreased the survival rate at 2 h to 33% (P < 0.05 vs. the saline-pre-treated group). CONCLUSIONS: The results support the hypothesis that histaminergic system activation leads to mobilisation of compensatory mechanisms in haemorrhagic hypotension.