Selective Histamine H3 Receptor Agonist Research Articles

Selective histamine H2 receptor agonists alleviate blood-brain barrier disruption by promoting the expression of vascular protective factors following traumatic brain injury in mice

Histamine is a major neurotransmitter and alleviates neuronal damage after ischemic injury via H2 receptors. Herein, we investigated the effects of H2 receptor agonists on the blood-brain barrier (BBB) disruption after traumatic brain injury (TBI). Male ddY mice were used to generate the TBI model, in which a fluid percussion injury (FPI) was induced by a hydraulic impact. The BBB disruption was evaluated using Evans blue extravasation. H2 receptor agonists, amthamine and dimaprit, were administered into the lateral cerebroventricle (i.c.v.) or tail vein (i.v.) from 3 hours to 3 days after FPI. The i.c.v. or i.v. administration of amthamine and dimaprit reduced FPI-induced Evans blue extravasation and promoted mRNA expression of vascular protective factors, including angiopoietin-1 and sonic hedgehog. The co-administration of ranitidine, a H2 receptor antagonist, inhibited these effects. Expression of the H2 receptor was observed in astrocytes and brain microvascular endothelial cells (BMECs) in the injured cortex. Treatment with amthamine and dimaprit promoted mRNA expression of vascular protective factors in astrocytes and BMECs. These results suggest that H2 receptor agonists alleviate TBI-induced BBB disruption by increasing the expression of vascular protective factors in astrocytes and BMECs.

RETRACTED: Hospital IoT big data and real-time monitoring and nursing intervention for patients with insomnia

Abstract Insomnia is a common problem that has been affecting residents for a long time. This study's main purpose of determining whether insomnia results from greater improvements in sleep using hypnosis than just the educational process of sleep hygiene. Sleep restriction therapy is a weekly session based on the following sleep hygiene education sessions. Provides an important aspect of assessing patients with insomnia and long-term care measures with an existing review of big data processing options. These include various non-drug therapy, drug therapy, cognitive behavioral therapy, the most important treatment for insomnia, benzodiazepines, and Z-drugs melatonin receptor agonist and a selective histamine H1 receptor agonist. A Deep Learning IoT including drugs, origin antagonists, antidepressants, antipsychotics, anticonvulsants and non-selective antihistamine. Many unsupervised data made a valuable tool for large data analysis, where the original data is mainly unmarked and unclassified. Complex patterns from sleeplessness, big data analysis, large amounts of data, semantic indexing, data annotation, and fast information retrieval to simplify identification tasks and some of the important things in the Internet of Things deeply explore the methods be used for problems. However, the deep basic learning array of drug therapy has the above characteristics of insomniac patients, and therapeutic research effects fully characterize its risk/benefit curve. These measures can form the basis of insomnia systems and evidence-based treatment in the clinical setting. Looking at the evidence base in this area and improving many patients' management in many hospitals highlights areas where insomnia needs to provide resources and be investigated.

Anticonvulsant effects of isomeric nonimidazole histamine H3 receptor antagonists.

Phenytoin (PHT), valproic acid, and modern antiepileptic drugs (AEDs), eg, remacemide, loreclezole, and safinamide, are only effective within a maximum of 70%–80% of epileptic patients, and in many cases the clinical use of AEDs is restricted by their side effects. Therefore, a continuous need remains to discover innovative chemical entities for the development of active and safer AEDs. Ligands targeting central histamine H3 receptors (H3Rs) for epilepsy might be a promising therapeutic approach. To determine the potential of H3Rs ligands as new AEDs, we recently reported that no anticonvulsant effects were observed for the (S)-2-(4-(3-(piperidin-1-yl)propoxy)benzylamino)propanamide (1). In continuation of our research, we asked whether anticonvulsant differences in activities will be observed for its R-enantiomer, namely, (R)-2-(4-(3-(piperidin-1-yl)propoxy)benzylamino)propaneamide (2) and analogs thereof, in maximum electroshock (MES)-, pentylenetetrazole (PTZ)-, and strychnine (STR)-induced convulsion models in rats having PHT and valproic acid (VPA) as reference AEDs. Unlike the S-enantiomer (1), the results show that animals pretreated intraperitoneally (ip) with the R-enantiomer 2 (10 mg/kg) were moderately protected in MES and STR induced models, whereas proconvulsant effect was observed for the same ligand in PTZ-induced convulsion models. However, animals pretreated with intraperitoneal doses of 5, 10, or 15 mg/kg of structurally bulkier (R)-enantiomer (3), in which 3-piperidinopropan-1-ol in ligand 2 was replaced by (4-(3-(piperidin-1-yl)propoxy)phenyl)methanol, and its (S)-enantiomer (4) significantly and in a dose-dependent manner reduced convulsions or exhibited full protection in MES and PTZ convulsions model, respectively. Interestingly, the protective effects observed for the (R)-enantiomer (3) in MES model were significantly greater than those of the standard H3R inverse agonist/antagonist pitolisant, comparable with those observed for PHT, and reversed when rats were pretreated with the selective H3R agonist R-(α)-methyl-histamine. Comparisons of the observed antagonistic in vitro affinities among the ligands 1–6 revealed profound stereoselectivity at human H3Rs with varying preferences for this receptor subtype. Moreover, the in vivo anticonvulsant effects observed in this study for ligands 1–6 showed stereoselectivity in different convulsion models in male adult rats.

Anticonvulsant and procognitive properties of the non-imidazole histamine H3 receptor antagonist DL77 in male adult rats

It has become clear that histamine H3 receptors (H3Rs) are implicated in modulating epilepsy and memory in laboratory animals. The new non-imidazole H3R antagonist DL77 has excellent selectivity profile and shows high in-vivo potency as well as in-vitro antagonist affinity with ED50 values of 2.1 ± 0.2 mg/kg and 8.4 ± 1.3 [nM], respectively. In the present study, the anticonvulsant effects of DL77 on maximal electroshock (MES)-, pentylenetetrazole (PTZ)-, and strychnine (STR)-induced seizure models were investigated. Moreover, the procognitive properties of DL77 were tested on acquisition, consolidation and retrieval processes in a one-trial inhibitory avoidance task in male Wistar rats. The results indicate that DL77 (5, 10, and 15 mg/kg, i.p.) significantly and dose-dependently reduced MES-induced seizure duration, whereas no protection was observed in PTZ- or STR-induced seizures. Importantly, the protective action observed for DL77 in MES-induced seizure was comparable to that of the reference antiepileptic drug (AED) phenytoin (PHT), and was also reversed when rats were pretreated with the CNS penetrant pyrilamine (PYR) (10 mg/kg, i.p.), or with the selective H3R agonist R-(α)-methyl-histamine (RAMH) (10 mg/kg, i.p.). Furthermore, the procognitive studies indicate that acute pre-training systemic administration of DL77 (2.5 mg/kg, i.p.) facilitated acquisition, whereas pre-testing acute administration of DL77 (5 and 10 mg/kg, i.p.) improved retrieval. Interestingly, the procognitive effect of DL77 on retrieval was completely abrogated when rats were pretreated with the centrally-acting H2R antagonist zolantidine (ZOL) but not the centrally acting H1R antagonist PYR, indicating that histaminergic pathways through activation of H2Rs appear to be participating in neuronal circuits involved in retrieval processes. Taken together, our results show that DL77 demonstrates anticonvulsant properties in the MES-induced seizure model and improves cognitive performance through actions on different memory stages. Therefore, H3Rs may have implications for the treatment of degenerative disorders associated with impaired memory function and may represent a novel therapeutic pharmacological target to tackle cognitive problems associated with the chronic use of antiepileptic drugs.This article is part of the Special Issue entitled ‘Histamine Receptors’.

High constitutive Akt2 activity in U937 promonocytes: effective reduction of Akt2 phosphorylation by the histamine H2-receptor and the β2-adrenergic receptor.

Histamine (HA) is approved for the treatment of acute myeloid leukemia (AML). Its antileukemic activity is related to histamine H2-receptor (H2R)-mediated inhibition of reactive oxygen species (ROS) production in myeloid cells facilitating survival of antineoplastic lymphocytes. The phosphatidylinositol 3-kinase (PI3K)/Akt signaling pathway, which plays a crucial role in cell survival and proliferation, is constitutively activated in leukemic cells of most AML patients resulting in poor survival prognosis. In a proof-of-principle experiment using a human phosphorylated mitogen-activated protein kinase (MAPK) array, we found high phosphorylation levels of Akt2 in U937 promonocytes that was abrogated by HA or selective H2R agonists. The H2R and the β2-adrenergic receptor (β2AR) are Gs-protein-coupled receptors. Stimulation results in adenylyl cyclase activation followed by generation of the second messenger adenosine 3′,5′-cyclic monophosphate (cAMP). In our present study, we evaluated the pharmacological profile of the H2R and the β2AR regarding Akt2 phosphorylation at Ser474 via western blot analysis and ELISA and cAMP accumulation via HPLC-MS/MS in U937 promonocytes. H2R and β2AR agonists concentration-dependently decreased Akt2 phosphorylation at Ser474. Deviations of potencies and efficacies of agonists in Akt2 phosphorylation and cAMP accumulation assays indicated participation of cAMP-independent signaling in GPCR-induced reduction of Akt2 phosphorylation. Accordingly, our study supports the concept of functional selectivity of the H2R and the β2AR in U937 promonocytes. In summary, we extended the antileukemic mechanism of HA via H2R and revealed the potential of β2AR agonists, which are already approved in the treatment of bronchial asthma and chronic obstructive pulmonary disease, as antileukemic drugs.

Pharmacological Evidence that Histamine H3 Receptors Mediate Histamine-Induced Inhibition of the Vagal Bradycardic Out-flow in Pithed Rats.

In vivo stimulation of cardiac vagal neurons induces bradycardia by acetylcholine (ACh) release. As vagal release of ACh may be modulated by autoreceptors (muscarinic M2 ) and heteroreceptors (including serotonin 5-HT1 ), this study has analysed the pharmacological profile of the receptors involved in histamine-induced inhibition of the vagal bradycardic out-flow in pithed rats. For this purpose, 180 male Wistar rats were pithed, artificially ventilated and pre-treated (i.v.) with 1mg/kg atenolol, followed by i.v. administration of physiological saline (1ml/kg), histamine (10, 50, 100 and 200μg/kg) or the selective histamine H1 (2-pyridylethylamine), H2 (dimaprit), H3 (methimepip) and H4 (VUF 8430) receptor agonists (1, 10, 50 and 100μg/kg each). Under these conditions, electrical stimulation (3, 6 and 9Hz; 15±3V and 1ms) of the vagus nerve resulted in frequency-dependent bradycardic responses, which were (i) unchanged during the infusions of saline, 2-pyridylethylamine, dimaprit or VUF 8430; and (ii) dose-dependently inhibited by histamine or methimepip. Moreover, the inhibition of the bradycardia caused by 50μg/kg of either histamine or methimepip (which failed to inhibit the bradycardic responses to i.v. bolus injections of acetylcholine; 1-10μg/kg) was abolished by the H3 receptor antagonist JNJ10181457 (1mg/kg, i.v.). In conclusion, our results suggest that histamine-induced inhibition of the vagal bradycardic out-flow in pithed rats is mainly mediated by pre-junctional activation of histamine H3 receptors, as previously demonstrated for the vasopressor sympathetic out-flow and the vasodepressor sensory CGRPergic (calcitonin gene-related peptide) out-flow.

The novel non-imidazole histamine H3 receptor antagonist DL77 reduces voluntary alcohol intake and ethanol-induced conditioned place preference in mice

It has become clear that histamine H3 receptors (H3R) have been implicated in modulating ethanol intake and preference in laboratory animals. The novel non-imidazole H3R antagonist DL77 with excellent selectivity profile shows high in-vivo potency as well as in-vitro antagonist affinity with ED50 of 2.1±0.2mg/kg and pKi=8.08, respectively. In the present study, and applying an unlimited access two-bottle choice procedure, the anti-alcohol effects of the H3R antagonist, DL77 (0, 3, 10 and 30mg/kg; i.p.), were investigated in adult mice. In this C57BL/6 line, effects of DL77 on voluntary alcohol intake and preference, as well as on total fluid intake were evaluated. Results have shown that DL77, dose-dependently, reduced both ethanol intake and preference. These effects were very selective as both saccharin and quinine, used to control for taste sensitivity, and intakes were not affected following DL77 pre-application. More importantly, systemic administration of DL77 (10mg/kg) during acquisition inhibited ethanol-induced conditioned-place preference (EtOH-CPP) as measured using an unbiased protocol. The anti-alcohol activity observed for DL77 was abrogated when mice were pretreated with the selective H3R agonist R-(α)-methyl-histamine (RAMH) (10mg/kg), or with the CNS penetrant H1R antagonist pyrilamine (PYR) (10mg/kg). These results suggest that DL77 has a predominant role in two in vivo effects of ethanol. Therefore, signaling via H3R is essential for ethanol-related consumption and conditioned reward and may represent a novel therapeutic pharmacological target to tackle ethanol abuse and alcoholism.

Flow cytometric analysis with a fluorescently labeled formyl peptide receptor ligand as a new method to study the pharmacological profile of the histamine H2 receptor.

The histamine H2 receptor (H2R) is a Gs protein-coupled receptor. Its activation leads to increases in the second messenger adenosine-3',5'-cyclic monophosphate (cAMP). Presently, several systems are established to characterize the pharmacological profile of the H2R, mostly requiring radioactive material, animal models, or human blood cells. This prompted us to establish a flow cytometric analysis with a fluorescently labeled formyl peptide receptor (FPR) ligand in order to investigate the H2R functionally and pharmacologically. First, we stimulated U937 promonocytes, which mature in a cAMP-dependent fashion upon H2R activation, with histamine (HA) or selective H2R agonists and measured increases in cAMP concentrations by mass spectrometry. Next, indicative for the maturation of U937 promonocytes, we assessed the FPR expression upon incubation with HA or H2R agonists. FPR expression was measured either indirectly by formyl peptide-induced changes in intracellular calcium concentrations ([Ca(2+)]i) or directly with the fluorescein-labeled FPR ligand fNleLFNleYK-Fl. HA and H2R agonists concentration-dependently induced FPR expression, and potencies and efficacies of fMLP-induced increases in [Ca(2+)]i and FPR density correlated linearly. Accordingly, flow cytometric analysis of FPR expression constitutes a simple, inexpensive, sensitive, and reliable method to characterize the H2R pharmacologically. Furthermore, we evaluated FPR expression at the mRNA level. Generally, quantitative real-time polymerase chain reaction confirmed functional data. Additionally, our study supports the concept of functional selectivity of the H2R, since we observed dissociations in the efficacies of HA and H2R agonists in cAMP accumulation and FPR expression.

Role of histamine H3 receptor in glucagon-secreting αTC1.6 cells

Pancreatic α-cells secrete glucagon to maintain energy homeostasis. Although histamine has an important role in energy homeostasis, the expression and function of histamine receptors in pancreatic α-cells remains unknown. We found that the histamine H3 receptor (H3R) was expressed in mouse pancreatic α-cells and αTC1.6 cells, a mouse pancreatic α-cell line. H3R inhibited glucagon secretion from αTC1.6 cells by inhibiting an increase in intracellular Ca2+ concentration. We also found that immepip, a selective H3R agonist, decreased serum glucagon concentration in rats. These results suggest that H3R modulates glucagon secretion from pancreatic α-cells.

(R)‐alpha‐methylhistamine Suppresses Inhibitory Neurotransmission in Hippocampal CA1 Pyramidal Neurons Counteracting Propofol‐Induced Amnesia in Rats

Propofol is a short-acting, intravenous general anesthetic that is widely used in clinical practice for short procedures; however, it causes depressed cognitive function for several hours thereafter. (R)-alpha-methylhistamine (RAMH), a selective histamine H3 receptor agonist, can enhance memory retention and attenuates memory impairment in rats. In this study, we investigated whether RAMH could rescue propofol-induced memory deficits and the underlying mechanisms partaking in this process. In the modified Morris water maze (MWM) test, rats were randomized into the following groups: control, propofol (25mg/kg, i.p., 30min before training), RAMH (10mg/kg, i.p., 60min before training), and propofol plus RAMH. All randomized rats were subjected to 2days of training, and a probe test was conducted on day 3. Field excitatory postsynaptic potentials were recorded from CA1 neurons in rat hippocampal slices, and long-term potentiation (LTP) was induced by either theta-burst stimulation (TBS) or high-frequency tetanic stimulation (HFS). Spontaneous and miniature inhibitory (sIPSCs, mIPSCs) or excitatory (sEPSCs, mEPSCs) postsynaptic currents were recorded from CA1 pyramidal neurons by whole-cell patch clamp. In the MWM task, propofol injection significantly impaired spatial memory retention. Pretreatment with RAMH reversed propofol-induced memory retention. In hippocampal CA1 slices, propofol perfusion markedly inhibited TBS- but not HFS-induced LTP. Co-perfusion of RAMH reversed the inhibitory effect of propofol on TBS-induced LTP reduction. Furthermore, in hippocampal CA1 pyramidal neurons, RAMH significantly suppressed the frequency but not the amplitude of sIPSCs and mIPSCs and had little effects on both the frequency and amplitude of sEPSCs and mEPSCs. Our results suggest that RAMH, by inhibiting presynaptic GABAergic neurotransmission, suppresses inhibitory neurotransmission in hippocampal CA1 pyramidal neurons, which in turn reverses inhibition of CA1 LTP and the spatial memory deficits induced by propofol in rats.

Differential effects of the histamine H(3) receptor agonist methimepip on dentate granule cell excitability, paired-pulse plasticity and long-term potentiation in prenatal alcohol-exposed rats.

We previously reported that prenatal alcohol-induced deficits in dentate gyrus (DG) long-term potentiation (LTP) are ameliorated by the histamine H3 receptor inverse agonist ABT-239. ABT-239 did not enhance LTP in control rats, suggesting that the possibility of a heightened H3 receptor-mediated inhibition of LTP in prenatal alcohol-exposed (PAE) offspring. To further investigate this mechanism, we examined the effect of methimepip, a selective histamine H3 receptor agonist, on DG granule cell responses and LTP in saccharin control and PAE rats. Long-Evans rat dams voluntarily consumed either a 0 or 5% ethanol solution 4 hours each day throughout gestation. Adult male offspring from these dams were anesthetized with urethane and electrodes implanted into the entorhinal cortical perforant path and the DG. In control offspring, methimepip reduced the coupling of fast excitatory postsynaptic field potentials to population spikes (E-S coupling), the probability of glutamate release, as measured by paired-pulse ratio (PPR) and diminished DG LTP. Similar reductions in E-S coupling and LTP were observed in saline-treated PAE offspring. In contrast to the control group, methimepip did not exacerbate PAE-induced reductions in E-S coupling or LTP. While the effects of methimepip in control offspring were consistent with speculation of a PAE-induced enhancement of H3 receptor-mediated inhibition of E-S coupling and LTP, the absence of an added effect of methimepip in PAE offspring could indicate either an inability to further inhibit these responses with methimepip in PAE rats or the presence of more complex regulatory neural interactions with in vivo recordings in PAE rats. Follow-up studies of H3 receptor-mediated responses in DG slice preparations are under way to provide clearer insights into the role of the H3 receptor regulation of excitatory transmission in PAE rats.

Anticonvulsant properties of histamine H3 receptor ligands belonging to N-substituted carbamates of imidazopropanol

Ligands targeting central histamine H3 receptors (H3Rs) for epilepsy might be a promising therapeutic approach. Therefore, the previously described and structurally strongly related imidazole-based derivatives belonging to carbamate class with high H3R in vitro affinity, in-vivo antagonist potency, and H3R selectivity profile were investigated on their anticonvulsant activity in maximal electroshock (MES)-induced and pentylenetetrazole (PTZ)-kindled seizure models in Wistar rats. The effects of systemic injection of H3R ligands 1–13 on MES-induced and PTZ-kindled seizures were screened and evaluated against the reference antiepileptic drug (AED) Phenytoin (PHT) and the standard histamine H3R inverse agonist/antagonist Thioperamide (THP) to determine their potential as new antiepileptic drugs. Following administration of the H3R ligands 1–13 (5, 10 and 15mg/kg, ip) there was a significant dose dependent reduction in MES-induced seizure duration. The protective action observed for the pentenyl carbamate derivative 4, the most protective H3R ligand among 1–13, was significantly higher (P <0.05) than that of standard H3R antagonist THP, and was reversed when rats were pretreated with the selective H3R agonist R-(α)-methyl-histamine (RAMH) (10mg/kg), or with the CNS penetrant H1R antagonist Pyrilamine (PYR) (10mg/kg). In addition, subeffective dose of H3R ligand 4 (5mg/kg, ip) significantly potentiated the protective action in rats pretreated with PHT (5mg/kg, ip), a dose without appreciable protective effect when given alone. In contrast, pretreatment with H3R ligand 4 (10mg/kg ip) failed to modify PTZ-kindled convulsion, whereas the reference drug PHT was found to fully protect PTZ-induced seizure. These results indicate that some of the investigated imidazole-based H3R ligands 1–13 may be of future therapeutic value in epilepsy.

Influence of the novel histamine H3 receptor antagonist ST1283 on voluntary alcohol consumption and ethanol-induced place preference in mice

Growing evidence supports a role for the central histaminergic system to have a modulatory influence on drug addiction in general and alcohol-use disorders in particular through histamine H3 receptors (H3R). In the present study, the effects of systemic injection of the newly synthesized H3R antagonist ST1283 on ethanol (EtOH) voluntary intake and EtOH-conditioned reward in mice have been investigated. Oral EtOH, saccharin, and quinine intake was assessed in a two-bottle choice paradigm using escalating concentrations of alcohol or tastant solutions. EtOH-induced place preference (CPP), EtOH-induced locomotor activity, and blood ethanol concentration (BEC) were also measured. Following administration of the H3R antagonist (2.5, 5, and 10 mg/kg, i.p.), there was a significant dose-dependent decrease in alcohol consumption and preference. Importantly, vehicle- and ST1283 (5 mg/kg)-treated mice showed similar consumption and preference to increasing concentration of both sweet and bitter tastes. More interestingly, systemic administration of ST1283 inhibited EtOH-CPP and EtOH-enhanced locomotion. This inhibition was blocked when mice were pretreated with the selective H3R agonist R-(alpha)-methyl-histamine (10 mg/kg). Finally, vehicle- and ST1283-treated mice had similar BECs. Our results show that ST1283 may decrease voluntary EtOH consumption and EtOH-CPP by altering its reinforcing effects, suggesting a novel role for histamine signaling in regulation of alcoholism. Lastly, the results add to the growing literature on H3R modulation in the pharmacotherapy of EtOH addiction.

Selective histamine H3 and H4 receptor agonists exert opposite effects against the gastric lesions induced by HCl in the rat stomach

The present study investigated the role of histamine H(3) and H(4) receptors in gastric mucosal defense, by the use of selective ligands. Firstly, the affinities of several histaminergic agonists for the rat histamine H(3) and H(4) receptors were checked in HEK 293T cells transfected with either receptor subtype. Next, functional activities were determined in conscious rat against the ulcerogenic effect of 0.6N HCl. Radioligand binding studies showed that immethridine and methimepip were the most selective agonists at rat H(3) receptors, whereas VUF10460 displayed approximately a 50-fold selectivity for the rat H(4) receptor over the H(3) receptor. In conscious rats, immethridine and methimepip significantly reduced (66% and 48% inhibition, respectively) the gastric lesions induced by HCl; the effect of immethridine was antagonized by the H(3) receptor antagonist A-331440, but not by the H(4) receptor antagonist JNJ7777120. The mixed H(3)/H(4) receptor agonist immepip induced a significant aggravation of HCl damage, which was prevented by JNJ7777120; HCl-induced lesions were also significantly enhanced by the H(4) receptor agonists VUF10460 and VUF8430; however, this effect was not modified by JNJ7777120. Overall, this study indicates that, whereas the histamine H(3) receptor is involved in the protection of rat stomach against concentrated HCl, the functional role of the H(4) receptor is still to be defined, although selective agonists induce proulcerogenic effects under HCl challenge. Finally, the species-dependent variations in affinity and receptor selectivity observed for most ligands need to be carefully addressed in the pharmacological characterization of histamine H(3) and H(4) receptor functions in vivo.

Proadrenergic effects of cardiac natriuretic peptides: inhibition by histamine H3‐receptor (H3R) activation

Natriuretic peptides (NP) are generally viewed as cardioprotective. Yet, NP were recently found to increase myocardial infarct size in mice, while their deletion decreased it. Also, mice lacking NP receptor-A are resistant to endotoxic shock. Further, a recombinant NP (nesiritide) was shown to increase death risk in heart failure patients. Excessive norepinephrine (NE) release may have been pivotal in these unexpected events. Langendorff guinea-pig hearts were perfused with NP; this resulted in a ≈ 73% increase in NE overflow. NP treatment of guinea-pig cardiac synaptosomes and NGF-differentiated PC12 cells also increased NE and dopamine release (≈ 34% and ≈ 48% increase, respectively). Thus, we investigated the mechanisms of the pro-adrenergic effect of NP. We found that NP markedly increased PKG activity, cAMP levels and PKA activity in differentiated PC12 cells; these effects derived from a cGMP/PKG-mediated prevention of cAMP hydrolysis by PDE3. Because of the antiexocytotic properties of H3R, we next questioned whether H3R activation might inhibit the proadrenergic effects of NP. Activation of H3R, constitutively expressed in heart synaptosomes and permanently transfected in PC12 cells, inhibited NP-induced catecholamine release and prevented the increases in cAMP level and PKA activity. Since the clinical use of NP in congestive heart failure (CHF) remains controversial, our findings may foster the design of selective H3R agonists to enable a safe and effective treatment of CHF with NP. This work was supported by NIH Grant HL 034215 and by a PhRMA Pre-doctoral fellowship.

Pre-synaptic histamine H3 receptors modulate glutamatergic transmission in rat globus pallidus

The globus pallidus, a neuronal nucleus involved in the control of motor behavior, expresses high levels of histamine H3 receptors (H3Rs) most likely located on the synaptic afferents to the nucleus. In this work we studied the effect of the activation of rat pallidal H3Rs on depolarization-evoked neurotransmitter release from slices, neuronal firing rate in vivo and turning behavior. Perfusion of globus pallidus slices with the selective H3R agonist immepip had no effect on the release of [3H]-GABA ([3H]-γ-aminobutyric acid) or [3H]-dopamine evoked by depolarization with high (20 mM) K+, but significantly reduced [3H]-d-aspartate release (−44.8±2.6% and −63.7±6.2% at 30 and 100 nM, respectively). The effect of 30 nM immepip was blocked by 10 μM of the selective H3R antagonist A-331440 (4′-[3-[(3(R)-dimethylamino-1-pyrrolidinyl]propoxy]-[1,1-biphenyl]-4′-carbonitrile). Intra-pallidal injection of immepip (0.1 μl, 100 μM) decreased spontaneous neuronal firing rate in anaesthetized rats (peak inhibition 68.8±10.3%), and this effect was reversed in a partial and transitory manner by A-331440 (0.1 μl, 1 mM). In free-moving rats the infusion of immepip (0.5 μl; 10, 50 and 100 μM) into the globus pallidus induced dose-related ipsilateral turning following systemic apomorphine (0.5 mg/kg, s.c.). Turning behavior induced by immepip (0.5 μl, 50 μM) and apomorphine was partially prevented by the local injection of A-331440 (0.5 μl, 1 mM) and was not additive to the turning evoked by the intra-pallidal injection of antagonists at ionotropic glutamate receptors (0.5 μl, 1 mM each of AP-5, dl-2-amino-5-phosphonovaleric acid, and CNQX, 6-nitro-7-sulphamoylbenzo[f]quinoxaline-2,3-dione). These results indicate that pre-synaptic H3Rs modulate glutamatergic transmission in rat globus pallidus and thus participate in the control of movement by basal ganglia.

A novel proadrenergic effect of cardiac natriuretic peptides: inhibition by histamine H3‐receptor activation

Natriuretic peptides (NP) are generally viewed as cardioprotective. Yet, NP were recently found to increase myocardial infarct size in mice, while their deletion decreased it. Further, a recombinant NP (nesiritide) was shown to increase death risk in heart failure patients. Excessive norepinephrine release may have been pivotal in these unexpected events. Thus, we questioned by which mechanisms NP could exert a pro‐adrenergic effect and whether this could be inhibited by histamine H3‐receptor (H3R) activation. NP treatment of sympathetic nerve endings isolated from guinea‐pig hearts (cardiac synaptosomes) and NGF‐differentiated PC12 cells markedly increased cAMP levels and PKA activity, thus promoting catecholamine exocytosis. These pro‐adrenergic effects derived from a cGMP/PKG‐mediated prevention of cAMP hydrolysis by PDE3. Activation of H3R, constitutively expressed in synaptosomes and permanently transfected in PC12 cells, inhibited NP‐induced catecholamine release and prevented the increases in cAMP levels and PKA activity. These effects most likely resulted from a H3R‐mediated enhancement of PDE3 activity and/or PKG inhibition. These findings elucidate new means to control catecholamine exocytosis in the heart, and may foster the design of new agents (e.g., selective H3R agonists) to enable a safe and effective treatment of congestive heart failure with NP.This work was supported by NIH Grant HL 034215 and by a PhRMA Pre‐doctoral fellowship.

Histamine excites rat lateral vestibular nuclear neurons through activation of post-synaptic H2 receptors

Through whole-cell patch recordings in brainstem slices, the effects of histamine on neuronal activity of the lateral vestibular nucleus (LVN) were investigated. Bath application of histamine elicited a concentration-dependent excitation of both spontaneous firing (n=19) and silent (n=7) LVN neurons. Moreover, histamine induced a stable inward current in the LVN neurons (n=5) and the histamine-induced depolarization of membrane potential persisted in the presence of tetrodotoxin (n=4), indicating a direct post-synaptic effect of the histamine on the LVN neurons. Selective histamine H2 receptor antagonist ranitidine effectively blocked the histamine-evoked excitatory responses on the LVN neurons (n=4), but selective histamine H1 receptor antagonist triprolidine did not (n=4). In addition, selective histamine H2 receptor agonist dimaprit (n=3) rather than 2-pyridylethylamine (n=4), a selective histamine H1 receptor agonist, mimicked the excitatory action of histamine on LVN neurons. The results demonstrate that histamine excites the LVN neurons via post-synaptic histamine H2 receptors and suggest that the central histaminergic projection arising from the hypothalamus may modulate LVN neurons activity and actively influence the vestibular reflexes and functions.

Excitatory effect of histamine on neuronal activity of rat cerebellar fastigial nucleus in vitro

The cerebellar fastigial nucleus (FN) holds an important role in motor control and body balance. Previous studies have revealed that the nucleus is innervated by direct hypothalamocerebellar histaminergic fibers. However, the functional role of histaminergic projection in cerebellar FN has never been established. In this study, we investigated the effect of histamine on neuronal firing of cerebellar FN by using slice preparations. Sixty-five FN cells were recorded from 47 cerebellar slices, and a vast majority of the cells responded to histamine stimulation with an excitatory response (58/65, 89.2%). Perfusing slices with low-Ca2+/high-Mg2+ medium did not block the histamine-induced excitation (n=10), supporting a direct postsynaptic action of histamine on the cells. Furthermore, the excitatory effect of histamine on FN neurons was not blocked by selective histamine H1 receptor antagonist triprolidine (n=15) or chlorpheniramine (n=10), but was effectively suppressed by ranitidine (n=15), a highly selective histamine H2 receptor antagonist. On the other hand, highly selective histamine H2 receptor agonist dimaprit (n=20) instead of histamine H1 receptor agonist 2-pyridylethylamine (n=16) mimicked the excitatory effect of histamine on FN neurons. The dimaprit-induced FN neuronal excitation was effectively antagonized by selective histamine H2 receptor antagonist ranitidine (n=13) but not influenced by selective histamine H1 receptor antagonist triprolidine (n=15). These results demonstrate that histamine excites cerebellar FN cells via the histamine H2 receptor mechanism and suggest that the hypothalamocerebellar histaminergic fibers may modulate cerebellar FN-mediated sensorimotor integration through their excitatory innervations on FN neurons.

Homologous down‐regulation of histamine H3 receptors in rat striatal slices

Preincubation of striatal slices with the selective histamine H3-receptor agonist immepip (100 nM) decreased the specific binding of N-alpha-[methyl-3H]-histamine ([3H]-NMHA) to membranes obtained from the treated slices. The binding decrease was significant after 5 min, remained at similar reduced levels between 5- and 30-min incubations with agonist, and only a partial recovery was observed after 90-min washout (34, 41, and 44% at 90, 120, and 150 min, respectively). Saturation analysis showed a significant decrease in both receptor density (-44% +/- 9%) and affinity (dissociation constant, Kd 1.15 +/- 0.23 nM from 0.59 +/- 0.17 nM). The effect of immepip was mimicked by histamine and the H3 agonists imetit and R-alpha-methylhistamine, and was blocked by the H3 antagonist thioperamide. The reduction in [3H]-NMHA binding was fully and partially prevented by incubation at 4 degrees C and in hypertonic medium, respectively, but not by the endocytosis inhibitor phenylarsine oxide (10 microM). None of the following protein kinase inhibitors, Ro-318220 and Gö-6976 (PKC), H-89 (PKA) and staurosporine (general inhibitor) prevented the effect of immepip. In [3H]-adenine-labeled slices the preincubation with immepip (100 nM, 15 min) prevented the inhibitory effect of H3 receptor activation on forskolin-induced [3H]-cAMP accumulation (99% +/- 9% vs. 76% +/- 4% of control values). Taken together our results indicate that agonist binding promotes the down-regulation of striatal H3 receptors resulting in a significant loss of function.