Histamine H3 Receptor Activation Research Articles

Histamine H3 receptor activation in the insular cortex during taste memory conditioning decreases appetitive response but accelerates aversive memory extinction under an ad libitum liquid regimen

Conditioned taste aversion (CTA) is a robust associative learning; liquid deprivation during this conditioning allows researchers to obtain readable measures of associative learning. Recent research suggests that thirst could be a crucial motivator that modulates conditioning and memory extinction processes, highlighting the importance of the body’s internal state during learning. Furthermore, the histaminergic system is one of the major modulatory systems controlling several behavioral and neurobiological functions, such as feeding, water intake, and nociception. Therefore, this research aimed to assess the effect of H3 histaminergic receptor activation in the insular cortex (IC) during CTA. For this, we conditioned adult male Wistar rats under two regimens: water deprivation and water ad libitum. A classical CTA protocol was used for water deprivation. Before CTA acquisition, 10 μM R-α-methylhistamine (RAMH), an H3 receptor agonist, was injected into the IC. Results showed that RAMH injections decreased CTA in water-deprived rats without affecting the significant aversion conditioning in rats that were given water ad libitum. Moreover, RAMH accelerated the process of aversive memory extinction under ad libitum water conditions. According to our findings, the degree of liquid satiety differentially affected taste-aversive memory formation, and H3 histamine receptors were more involved under water deprivation conditions during acquisition. However, these receptors modulated the strength of aversive conditioning by altering the rate of aversive memory extinction in the absence of deprivation. In conclusion, histaminergic activity in the IC may influence taste memory dynamics through different mechanisms depending on the degree of liquid satiety or deprivation during conditioning.

Histamine H2-receptor antagonism improves conduit artery endothelial function and reduces plasma aldosterone level without lowering arterial blood pressure in angiotensin II–hypertensive mice

Aldosterone through the mineralocorticoid receptor MR has detrimental effects on cardiovascular disease. It reduces the bioavailability of nitric oxide and impairs endothelium-dependent vasodilatation. In resistance arteries, aldosterone impairs the sensitivity of vascular smooth muscle cells to nitric oxide by promoting the local secretion of histamine which activates H2 receptors. The present experiments tested in vivo and ex vivo the hypothesis that systemic H2-receptor antagonism reduces arterial blood pressure and improves vasodilatation in angiotensin II–induced chronic hypertension. Hypertension was induced by intravenous infusion of angiotensin II (60 ng kg−1 min−1) in conscious, unrestrained mice infused concomitantly with the H2-receptor antagonist ranitidine (27.8 µg kg−1 min−1) or vehicle for 24 days. Heart rate and arterial blood pressure were recorded by indwelling arterial catheter. Resistance (mesenteric) and conductance (aortae) arteries were harvested for perfusion myography and isometric tension recordings by wire myography, respectively. Plasma was analyzed for aldosterone concentration. ANGII infusion resulted in elevated arterial blood pressure and while in vivo treatment with ranitidine reduced plasma aldosterone concentration, it did not reduce blood pressure. Ranitidine improved ex vivo endothelial function (acetylcholine 10−9 to 10−6 mol L−1) in mesenteric resistance arteries. This was abolished by ex vivo treatment with aldosterone (10−9 mol L−1, 1 h). In aortic segments, in vivo ranitidine treatment impaired relaxation. Activation of histamine H2 receptors promotes aldosterone secretion, does not affect arterial blood pressure, and protects endothelial function in conduit arteries but promotes endothelial dysfunction in resistance arteries during angiotensin II–mediated hypertension. Aldosterone contributes little to angiotensin II–induced hypertension in mice.

Computational Analysis of Histamine Protonation Effects on H1R Binding.

Despite numerous studies investigating histamine and its receptors, the impact of histamine protonation states on binding to the histamine H1-receptor (H1R) has remained elusive. Therefore, we assessed the influence of different histamine tautomers (τ-tautomer, π-tautomer) and charge states (mono- vs. dicationic) on the interaction with the ternary histamine-H1R-Gq complex. In atomistic molecular dynamics simulations, the τ-tautomer formed stable interactions with the receptor, while the π-tautomer induced a rotation of the histamine ring by 180° and formed only weaker hydrogen bonding interactions. This suggests that the τ-tautomer is more relevant for stabilization of the active ternary histamine-H1R-Gq complex. In addition to the two monocationic tautomers, the binding of dicationic histamine was investigated, whose interaction with the H1R had been observed in a previous experimental study. Our simulations showed that the dication is less compatible with the ternary histamine-H1R-Gq complex and rather induces an inactive conformation in the absence of the Gq protein. Our data thus indicate that the charge state of histamine critically affects its interactions with the H1R. Ultimately these findings might have implications for the future development of new ligands that stabilize distinct H1R activation states.

Chronic H3R activation reduces L-Dopa-induced dyskinesia, normalizes cortical GABA and glutamate levels, and increases striatal dopamine D1R mRNA expression in 6-hydroxydopamine-lesioned malerats.

Dyskinesias induced by L-3,4-dihydroxyphenylalanine, L-Dopa (LIDs), are the major complication in the pharmacological treatment of Parkinson's disease. LIDs induce overactivity of the glutamatergic cortico-striatal projections, and drugs that reduce glutamatergic overactivity exert antidyskinetic actions. Chronic administration of immepip, agonist at histamine H3 receptors (H3R), reduces LIDs and diminishes GABA and glutamate content in striatal dialysates (Avila-Luna et al., Psychopharmacology 236: 1937-1948, 2019). In rats unilaterally lesioned with 6-hydroxydopamine in the substantia nigra pars compacta (SNc), we examined whether the chronic administration of immepip and their withdrawal modify LIDs, the effect of L-Dopa on glutamate and GABA content, and mRNA levels of dopamine D1 receptors (D1Rs) and H3Rs in the cerebral cortex and striatum. The administration of L-Dopa for 21 days induced LIDs. This effect was accompanied by increased GABA and glutamate levels in the cerebral cortex ipsi and contralateral to the lesioned SNc, and immepip administration prevented (GABA) or reduced (glutamate) these actions. In the striatum, GABA content increased in the ipsilateral nucleus, an effect prevented by immepip. L-Dopa administration had no significant effects on striatal glutamate levels. In lesioned and L-Dopa-treated animals, D1R mRNA decreased in the ipsilateral striatum, an effect prevented by immepip administration. Our results indicate that chronic H3R activation reduces LIDs and the overactivity of glutamatergic cortico-striatal projections, providing further evidence for an interaction between D1Rs and H3Rs in the cortex and striatum under normal and pathological conditions.

Monitoring of histamine-induced calcium channel activity of a single cell using semiconducting carbon nanotube transistors

A method using transistors based on semiconducting carbon nanotubes were developed for the real-time monitoring of the electrophysiological responses of individual cells to histamine stimulation. Transistors with one or three floating electrodes were utilized to evaluate histamine-induced Ca2+ influx into Hela cells via the recording of the conductance changes of the transistors. The Ca2+ influx resulted from the activation of histamine H1 receptors embedded on the cell membranes by histamine, which generated a temporary negative potential at the gap between the cell and the transistor. Moreover, the antihistamine effects of chlorpheniramine on histamine-induced Ca2+ influx were also investigated by using a transistor including three floating electrodes. Especially, only a single transistor was applied to repeat the measurements of the responses of multiple Hela cells pretreated with chlorpheniramine to histamine stimulation. This allows us to acquire data without being suffered from device-to-device variations, implying our method would be a simple but powerful method for applications of nanoscale biosensors to electrophysiological studies.

The histamine H3 receptor modulates dopamine D2 receptor–dependent signaling pathways and mouse behaviors

The histamine H3 receptor (H3R) is highly enriched in the spiny projection neurons (SPNs) of the striatum, in both the D1 receptor (D1R)-expressing and D2 receptor (D2R)-expressing populations. A crossantagonistic interaction between H3R and D1R has been demonstrated in mice, both at the behavioral level and at the biochemical level. Although interactive behavioral effects have been described upon coactivation of H3R and D2R, the molecular mechanisms underlying this interaction are poorly understood. Here, we show that activation of H3R with the selective agonist R-(-)-α-methylhistamine dihydrobromide mitigates D2R agonist-induced locomotor activity and stereotypic behavior. Using biochemical approaches and the proximity ligation assay, we demonstrated the existence of an H3R-D2R complex in the mouse striatum. In addition, we examined consequences of simultaneous H3R-D2R agonism on the phosphorylation levels of several signaling molecules using immunohistochemistry. H3R agonist treatment modulated Akt (serine/threonine PKB)-glycogen synthase kinase 3 beta signaling in response to D2R activation via a β-arrestin 2-dependent mechanism in D2R-SPNs but not in D1R-SPNs. Phosphorylation of mitogen- and stress-activated protein kinase 1 and rpS6 (ribosomal protein S6) was largely unchanged under these conditions. As Akt-glycogen synthase kinase 3 beta signaling has been implicated in several neuropsychiatric disorders, this work may help clarify the role of H3R in modulating D2R function, leading to a better understanding of pathophysiology involving the interaction between histamine and dopamine systems.

Histamine via histamine H1 receptor enhances the muscarinic receptor-induced calcium response to acetylcholine in an enterochromaffin cell model.

As a prerequisite for serotonin secretion, the P‐STS ileal enterochromaffin cell line responds to acetylcholine (ACh) stimulation with an increase in intracellular calcium mediated by the muscarinic ACh receptor M3 (M3R). Histamine increases intracellular calcium via histamine H1 receptor (H1R) in P‐STS cells and pre‐incubation with histamine specifically augments the response to ACh but not to epinephrine or nicotine. We aimed to elucidate whether histamine receptors are involved in this synergism. Astonishingly, HEK‐293 T cells—known to express M3R, but only a very low amount of histamine receptor messenger RNA—showed a similar enhancement of the calcium response to ACh by pre‐incubation with histamine. Despite the much lower level of H1R protein detected in HEK‐293 T cells as compared to P‐STS cells, in both cell lines pre‐treatment with H1R antagonists inhibited the synergism between histamine and ACh. No indication for an involvement of histamine H2 or H4 receptors in the synergism was found. Furthermore, pre‐incubation with the cAMP‐inducing compound forskolin had no influence on the intracellular calcium response to ACh. Serotonin secretion from P‐STS cells was increased after challenge with ACh and histamine added simultaneously compared to ACh alone, suggesting that histamine increases ACh‐induced serotonin secretion from enterochromaffin cells. In conclusion, our data suggest that histamine enhances the M3R‐mediated intracellular calcium response to ACh via activation of H1R. This probably increases serotonin secretion from enterochromaffin cells and thereby affects intestinal motility in histamine intolerance, food allergies and irritable bowel syndrome.

Hypothesis: Amelioration of obesity-induced cognitive dysfunction via a lorcaserin-betahistine combination treatment.

The prolonged exposure to obesogenic diets disrupts the mesocortical dopaminergic input to the prefrontal cortex (PFC). This leads to suboptimal dopamine levels in this brain region, which affects cognition and control of food intake. Treatments that restore mesocortical dopaminergic neurotransmission may improve obesity‐associated cognitive dysfunction and modulate food intake to induce weight loss. Given the complexity and multifactorial nature of obesity, combination treatments would likely achieve sizeable and sustained body weight loss and improve obesity‐linked outcomes, such as cognitive dysfunction. Given this background, we hypothesize that concomitant activation of serotonin 5‐HT2C and histamine H1 receptors, coupled with antagonism of histamine H3 receptors, synergistically modulates mesocortical dopamine neurotransmission and ameliorates obesity‐induced cognitive dysfunction. We propose to test the hypothesis in a diet‐induced obesity (DIO) rat model by treating animals with the 5‐HT2C agonist lorcaserin and the H1 agonist and H3 antagonist betahistine. Consistent with our hypothesis, both lorcaserin and betahistine have been shown to reduce body weight in humans with obesity and animals. Both drugs have been demonstrated to improve cognitive functions by influencing dopaminergic signaling in the PFC. The proposed combination treatment addresses the paucity of studies on obesity treatments that improve cognitive function. This research may also help identify a potential targetable mechanism connecting obesity and neurocognitive outcomes.

Involvement of histamine H3 receptor agonism in premature ejaculation revealed by a novel evaluation system

ABSTRACT Introduction Premature ejaculation (PE) has been defined by the International Society for Sexual Medicine (ISSM) as follows: men with lifelong and acquired PE appear to share the dimensions of short ejaculatory latency, reduced or absent perceived ejaculatory control, and the presence of negative personal consequences. Although local anesthetics and serotonin selective reuptake inhibitors (SSRIs) have been mainly used for the treatment of PE, there are several safety concerns and weak efficacy. To date, there have been no pharmacological treatments for PE approved by the Food and Drug Administration (FDA) in the United States. Therefore, there is still a need for the development of novel therapeutics that have sufficient efficacy and fewer side effects. We identified Histamine H3 receptor (H3R) as a potential target for treatment of PE. It is well known that H3R is mainly expressed by neuronal cells of the central nervous system, but its expression on peripheral nerves, especially Aβ sensory nerves, has also been reported. Activation of H3R in peripheral sensory neurons has been reported to inhibit mechanical stimulation-induced pain. This suggests that H3R stimulation can suppress neural activity induced by mechanical stimulation in the penis, but there have been no reports on the role of H3R in ejaculation function, and its mechanism of action is unknown. Objective In order to characterize the role of H3R in ejaculation prolongation, electrophysiological and behavioral studies were performed with imetit (H3R agonist) and ciproxifan (H3R antagonist) in male rats. Methods We developed a novel electrophysiological evaluation system for analyzing the spinal-penile neurotransmission mechanism, investigated the effects of imetit on penile neurotransmission. Also, we evaluated the effect of imetit on mechanical stimuli induced neuronal firing at genitofemoral nerve. For evaluating the effects of imetit for ejaculation, we used the copulatory behavior test in order to directly measure ejaculation latency (EL), which is a primary endpoint in clinical trial. Results In vivo electrophysiological study, imeit significantly attenuated the mechanical stimuli induced firing at spinal dorsal horn in a novel evaluation system. This inhibitory effect of imetit was also observed at genitofemoral nerve. Inhibition on firing was significantly inhibited by ciproxifan, but not inhibited by H4R antagonist. In the copulatory behavior test, imetit showed a strong prolongation effect on EL. This effect was significantly inhibited by ciproxifan. Conclusions These results suggest that stimulation of H3R suppresses the mechanical stimuli induced neuronal firing on spinal-penile neurotransmission system, resulting in prolongation of ejaculation latency. H3R agonist is expected as a new treatment for PE. Disclosure Work supported by industry: yes, by Mitsubishi Tanabe Pharma Corporation. A consultant, employee (part time or full time) or shareholder is among the authors (Mitsubishi Tanabe Pharma Corporation).

Histamine Potentiates SARS-CoV-2 Spike Protein Entry Into Endothelial Cells.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) which causes coronavirus disease (COVID-19) is one of the most serious global health crises in recent history. COVID-19 patient symptoms range from life-threatening to mild and asymptomatic, which presents unique problems in identifying, quarantining, and treating the affected individuals. The emergence of unusual symptoms among survivors, now referred to as “Long COVID”, is concerning, especially since much about the condition and the treatment of it is still relatively unknown. Evidence so far also suggests that some of these symptoms can be attributed to vascular inflammation. Although famotidine, the commonly used histamine H2 receptor (H2R) blocker, was shown to have no antiviral activity, recent reports indicate that it could prevent adverse outcomes in COVID-19 patients. Histamine is a classic proinflammatory mediator, the levels of which increase along with other cytokines during COVID-19 infection. Histamine activates H2R signaling, while famotidine specifically blocks H2R activation. Investigating the effects of recombinant SARS-CoV-2 spike protein S1 Receptor-Binding Domain (Spike) on ACE2 expression in cultured human coronary artery endothelial cells, we found that the presence of histamine potentiated spike-mediated ACE2 internalization into endothelial cells. This effect was blocked by famotidine, protein kinase A inhibition, or by H2 receptor protein knockdown. Together, these results indicate that histamine and histamine receptor signaling is likely essential for spike protein to induce ACE2 internalization in endothelial cells and cause endothelial dysfunction and that this effect can be blocked by the H2R blocker, famotidine.

Anti-Allergic Effects of Myrciaria dubia (Camu-Camu) Fruit Extract by Inhibiting Histamine H1 and H4 Receptors and Histidine Decarboxylase in RBL-2H3 Cells.

Although Myrciaria dubia (camu-camu) has been shown to exert anti-oxidant and anti-inflammatory effects in both in vitro and in vivo studies, its use in allergic responses has not been elucidated. In the present study, the anti-allergic effect of 70% ethanol camu-camu fruit extract was tested on calcium ionophore (A23187)-induced allergies in RBL-2H3 cells. The RBL-2H3 cells were induced with 100 nM A23187 for 6 h, followed by a 1 h camu-camu fruit extract treatment. A23187 sanitization exacerbated mast cell degranulation; however, camu-camu fruit extract decreased the release of histamine and β-hexosaminidase, which are considered as key biomarkers in cell degranulation. Camu-camu fruit extract inhibited cell exocytosis by regulating the calcium/nuclear factor of activated T cell (NFAT) signaling. By downregulating the activation of mitogen-activated protein kinase (MAPK) signaling, camu-camu fruit extract hindered the activation of both histamine H1 and H4 receptors and inhibited histidine decarboxylase (HDC) expression by mediating its transcription factors KLF4/SP1 and GATA2/MITF. In A23187-induced ROS overproduction, camu-camu fruit extract activated nuclear factor erythroid-2-related factor 2 (Nrf2) to protect mast cells against A23187-induced oxidative stress. These findings indicate that camu-camu fruit extract can be developed to act as a mast cell stabilizer and an anti-histamine. This work also “opens the door” to new investigations using natural products to achieve breakthroughs in allergic disorder treatment.

Histamine H3 Receptor Signaling Regulates the NLRP3 Inflammasome Activation in C2C12 Myocyte During Myogenic Differentiation.

NLRP3 inflammasome has been implicated in impaired post-injury muscle healing and in muscle atrophy. Histamine receptors play an important role in inflammation, but the role of histamine H3 receptor (H3R) in myocyte regeneration and in the regulation of NLRP3 inflammasome is not known. We studied the effects of H3R signaling on C2C12 myocyte viability, apoptosis, and tumor necrosis factor alpha (TNFα)-induced NLRP3 inflammasome activation during striated myogenic differentiation at three time points (days 0, 3, and 6). Expression of Nlrp3, interleukin-1β (IL-1β), and myogenesis markers were determined. TNFα reduced overall viability of C2C12 cells, and exposure to TNFα induced apoptosis of cells at D6. Activation of H3R had no effect on viability or apoptosis, whereas inhibition of H3R increased TNFα-induced apoptosis. Stimulation of C2C12 cells with TNFα increased Nlrp3 mRNA expression at D3 and D6. Moreover, TNFα reduced the expression of myogenesis markers MyoD1, Myogenin, and Myosin-2 at D3 and D6. H3R attenuated TNFα-induced expression of Nlrp3 and further inhibited the myogenesis marker expression; while H3R -blockage enhanced the proinflammatory effects of TNFα and increased the myogenesis marker expression. TNFα-induced secretion of mature IL-1β was dependent on the activation of the NLRP3 inflammasome, as shown by the reduced secretion of mature IL-1β upon treatment of the cells with the small molecule inhibitor of the NLRP3 inflammasome (MCC950). The activation of H3R reduced TNFα-induced IL-1β secretion, while the H3R blockage had an opposite effect. In conclusion, the modulation of H3R activity regulates the effects of TNFα on C2C12 myocyte differentiation and TNFα-induced activation of NLRP3 inflammasome. Thus, H3R signaling may represent a novel target for limiting postinjury muscle inflammation and muscle atrophy.

Analysis of Missense Variants in the Human Histamine Receptor Family Reveals Increased Constitutive Activity of E4106.30×30K Variant in the Histamine H1 Receptor.

The Exome Aggregation Consortium has collected the protein-encoding DNA sequences of almost 61,000 unrelated humans. Analysis of this dataset for G protein-coupled receptor (GPCR) proteins (available at GPCRdb) revealed a total of 463 naturally occurring genetic missense variations in the histamine receptor family. In this research, we have analyzed the distribution of these missense variations in the four histamine receptor subtypes concerning structural segments and sites important for GPCR function. Four missense variants R1273.52×52H, R13934.57×57H, R4096.29×29H, and E4106.30×30K, were selected for the histamine H1 receptor (H1R) that were hypothesized to affect receptor activity by interfering with the interaction pattern of the highly conserved D(E)RY motif, the so-called ionic lock. The E4106.30×30K missense variant displays higher constitutive activity in G protein signaling as compared to wild-type H1R, whereas the opposite was observed for R1273.52×52H, R13934.57×57H, and R4096.29×29H. The E4106.30×30K missense variant displays a higher affinity for the endogenous agonist histamine than wild-type H1R, whereas antagonist affinity was not affected. These data support the hypothesis that the E4106.30×30K mutation shifts the equilibrium towards active conformations. The study of these selected missense variants gives additional insight into the structural basis of H1R activation and, moreover, highlights that missense variants can result in pharmacologically different behavior as compared to wild-type receptors and should consequently be considered in the drug discovery process.

Molecular Signaling and Transcriptional Regulation of Histamine H1 Receptor Gene.

Histamine-activated histamine H1 receptor (H1R) signaling regulates many gene expressions, mainly through the protein kinase C (PKC)/extracellular signal-regulated kinases (ERK) signaling. Involvement of other signaling, including NF-κB, Wnt, RUNX-2, and Rho A signaling was also demonstrated. In addition, cAMP production through the activation of H1R signaling was reported. H1R gene itself is also up-regulated by the activation of H1R signaling with histamine. Here, we review our recent findings in the molecular signaling and transcriptional regulation of the H1R gene. Stimulation with histamine up-regulates H1R gene expression through the activation of H1R in HeLa cells. The PKCδ/ERK/poly(ADP)ribosyl transferase-1 (PARP-1) signaling was involved in this up-regulation. Heat shock protein 90 also plays an important role in regulating PKCδ translocation. Promoter analyses revealed the existence of two promoters in the human H1R gene in HeLa cells. H1R-activated H1R gene up-regulation in response to histamine was also observed in U373 astroglioma cells. However, this up-regulation was mediated not through the PKCδ signaling but possibly through the PKCα signaling. In addition, the promoter region responsible for histamine-induced H1R gene transcription in U373 cells was different from that of HeLa cells. These findings suggest that the molecular signaling and transcriptional regulation of the H1R gene are different between neuronal cells and non-neuronal cells.

Histamine H3 Receptor Activation Modulates Glutamate Release in the Corticostriatal Synapse by Acting at CaV2.1 (P/Q-Type) Calcium Channels and GIRK (KIR3) Potassium Channels

The striatum is innervated by histaminergic fibers and expresses a high density of histamine H3 receptors (H3Rs), present on medium spiny neurons (MSNs) and corticostriatal afferents. In this study, in sagittal slices from the rat dorsal striatum, excitatory postsynaptic potentials (EPSPs) were recorded in MSNs after electrical stimulation of corticostriatal axons. The effect of H3R activation and blockers of calcium and potassium channels was evaluated with the paired-pulse facilitation protocol. In the presence of the H3R antagonist/inverse agonist clobenpropit (1μM), the H3R agonist immepip (1μM) had no effect on the paired-pulse ratio (PPR), but in the absence of clobenpropit, immepip induced a significant increase in PPR, accompanied by a reduction in EPSP amplitude, suggesting presynaptic inhibition. The blockade of CaV2.1 (P/Q-type) channels with ω-agatoxin TK (400nM) increased PPR and prevented the effect of immepip. The CaV2.2 (N-type) channel blocker ω-conotoxin GVIA (1μM) also increased PPR, but did not occlude the immepip action. Functional KIR3 channels are present in corticostriatal terminals, and in experiments in which immepip increased PPR, the KIR3 blocker tertiapin-Q (30nM) prevented the effect of the H3R agonist. These results indicate that the presynaptic modulation by H3Rs of corticostriatal synapses involves the inhibition of Cav2.1 calcium channels and the activation of KIR3 potassium channels.

Elucidation of Inverse Agonist Activity of Bilastine.

H1-antihistamines antagonize histamine and prevent it from binding to the histamine H1 receptor (H1R). Some of them also act as inverse agonists, which are more potent than pure antagonists because they suppress the constitutive H1R activity. Bilastine is a non-sedative antihistamine which is one of the most satisfy the requirements for oral antihistamines. However, there is no information to show the inverse agonist activity of bilastine including inositol phosphates accumulation, and its inverse agonist activity is yet to be elucidated. Here we evaluated whether bilastine has inverse agonist activity or not. Intracellular calcium concentration was measured using Fluo-8. Inositol phosphates accumulation was assayed using [3H]myo-inositol. The H1R mRNA level was measured using real-time RT-PCR. At rest, Ca2+ oscillation was observed, indicating that H1R has intrinsic activity. Bilastine attenuated this fluorescence oscillation. Bilastine suppressed the increase in IPs formation in a dose-dependent manner and it was about 80% of the control level at the dose of 3 μM. Bilastine also suppressed histamine-induced increase in IPs formation to the control level. Furthermore, bilastine suppressed basal H1R gene expression in a dose-dependent manner. Data suggest that bilastine is an inverse agonist. Preseasonal prophylactic administration with bilastine could down-regulate basal H1R gene expression in the nasal mucosa and ameliorate the nasal symptoms during the peak pollen period.

Effect of Histamine‐receptor Antagonism on VO2Peak Improvements to Exercise Training

One of the physiological changes following moderate intensity aerobic exercise is a transient reduction in blood pressure, termed post‐exercise hypotension (PEH), which is exercise intensity and duration dependent. This short‐term reduction in blood pressure is dependent upon skeletal muscle histamine production and release, which causes increased conductance via local activation of histamine H1 and H2 receptors. PEH is abolished when antihistamines are consumed prior to exercise. Additionally, histamine‐receptor antagonism during a single bout of exercise modifies expression of the transcriptome related to inflammation, angiogenesis, and metabolism. Due to the large role histamine plays in post‐exercise recovery, the scope of histamine receptor activation in adaptations to aerobic exercise training needs to be determined. Therefore, the purpose of this study was to examine histamine’s role in promoting adaptation to exercise. It is hypothesized that blocking histamine’s actions will attenuate adaptations that occur from aerobic exercise training. 16 young, healthy, recreationally active individuals (5M, 11F) volunteered for this study. All volunteers completed 6 weeks of exercise training (3–4 times a week), for a total of 21 exercise sessions. Exercise training combined 18 sessions of continuous exercise (1 hour at 60% VO2peak) and 3 sessions of high intensity interval training (30 min between 30 and 90% VO2peak). Volunteers were randomly placed into a placebo or histamine antagonism (540 mg fexofenadine; a H1‐receptor antagonist, and 300 mg of ranitidine; a H2‐receptor antagonist) group, and consumed medication 1 hour prior to each exercise session. Data was collected 4 times throughout the study with pre and post exercise training measures, as well as after every 2 weeks of exercise training. Measurements of VO2peak and cycle ergometry work rate were obtained. Exercise training resulted in increased absolute VO2peak of all subjects (9.4±1.5%; P<0.01); however, no difference was seen between groups (10.9±1.8% control vs 7.9±2.4% blockade; P=0.35). Additionally, exercise training resulted in increased cycle ergometry work rate of all subjects (11.2±1.6%; P<0.01); however, there was a trend for blunted improvement with blockade (7.7±2.4%) vs control (14.6±2.8%; P=0.08). In conclusion, blocking histamine’s actions had no effect on VO2 adaptations to exercise training. Additional information about factors related to O2 delivery and consumption are needed to understand the relationship between histamine and cardiovascular adaptation to exercise training.Support or Funding InformationSupport provided by: The Eugene & Clarissa Evonuk Memorial Graduate Fellowship

Effects of corticosteroid on mRNA levels of histamine H1 receptor in nasal mucosa of healthy participants and HeLa cells.

The purpose of this study is to examine the effect of intranasal corticosteroid (INCS) administration on histamine H1 receptor (H1R) gene expression in the nasal mucosa of healthy participants and the effects of dexamethasone on basal and histamine-induced H1R mRNA expression, and histamine-induced phosphorylation of extracellular signal-regulated kinase (ERK) in HeLa cells. Sixteen healthy participants were given INCS once daily for a week. After pretreatment of dexamethasone, HeLa cells were treated with histamine. Levels of H1R mRNA and phosphorylation of ERK were measured using real time PCR and immunoblot analysis, respectively. Levels of H1R mRNA in the nasal mucosa of healthy participants receiving INCS was significantly decreased. Dexamethasone suppressed basal levels of H1R mRNA, and histamine-induced up-regulation of H1R mRNA and ERK phosphorylation in HeLa cells. These data suggested that corticosteroid inhibited both basal transcription and histamine-induced transcriptional activation of H1R through its suppression of ERK phosphorylation in the signaling pathway involved in H1R gene transcription. It is further suggested that pre-seasonal prophylactic administration of INCS suppresses both basal and pollen-induced upregulation of H1R gene expression in the nasal mucosa of patients with pollinosis, leading to prevention of the exacerbation of nasal symptoms during peak pollen season. J. Med. Invest. 67 : 311-314, August, 2020.

Histamine H1 and H3 receptor activation increases the expression of Glucose Transporter 1 (GLUT-1) in rat cerebro-cortical astrocytes in primary culture

Astrocytes take up glucose via the 45 kDa isoform of the Glucose Transporter 1 (GLUT-1), and in this work we have investigated whether histamine regulates GLUT-1 expression in rat cerebro-cortical astrocytes in primary culture. Cultured astrocytes expressed histamine H1 and H3 receptors (H1Rs and H3Rs) as evaluated by radioligand binding. Receptor functionality was confirmed by the increase in the intracellular concentration of Ca2+ (H1R) and the inhibition of forskolin-induced cAMP accumulation (H3R). Quantitative RT-PCR showed that histamine and selective H1R and H3R agonists (1 h incubation) significantly increased GLUT-1 mRNA to 153 ± 7, 163 ± 2 and 168 ± 13% of control values, respectively. In immunoblot assays, incubation (3 h) with histamine or H1R and H3R agonists increased GLUT-1 protein levels to 224 ± 12, 305 ± 11 and 193 ± 13% of control values, respectively, an action confirmed by inmunocytochemistry. The effects of H1R and H3R agonists were blocked by the selective antagonists mepyramine (H1R) and clobenpropit (H3R). The pharmacological inhibition of protein kinase C (PKC) prevented the increase in GLUT-1 protein induced by either H1R or H3R activation. Furthermore, histamine increased ERK-1/2 phosphorylation, and the effect of H1R and H3R activation on GLUT-1 protein levels was reduced or prevented, respectively, by MEK-1/2 inhibition. These results indicate that by activating H1Rs and H3Rs histamine regulates the expression of GLUT-1 by astrocytes. The effect appears to involve the phospholipase C (PLC) → diacylglycerol (DAG)/Ca2+→ PKC and PLC → DAG/Ca2+ → PKC → MAPK pathways.

Histamine H3 receptor activation reduces the impairment in prepulse inhibition (PPI) of the acoustic startle response and Akt phosphorylation induced by MK-801 (dizocilpine), antagonist at N-Methyl-d-Aspartate (NMDA) receptors

We have investigated the effect of the local activation of histamine H3 receptors (H3Rs) in the rat prefrontal cortex (PFCx) on the impairment of pre-pulse inhibition (PPI) of the startle response induced by the systemic administration of MK-801, antagonist at glutamate N-Methyl-d-Aspartate (NMDA) receptors, and the possible functional interaction between H3Rs and MK-801 on PFCx dopaminergic transmission. Infusion of the H3R agonist RAMH (19.8 ng/1 μl) into the PFCx reduced or prevented the inhibition by MK-801 (0.15 mg/kg, ip) of PPI evoked by different auditory stimulus intensities (5, 10 and 15 dB), and the RAMH effect was blocked by the H3R antagonist/inverse agonist ciproxifan (30.6 ng/1 μl). MK-801 inhibited [3H]-dopamine uptake (−45.4 ± 2.1%) and release (−32.8 ± 2.6%) in PFCx synaptosomes or slices, respectively, and molecular modeling indicated that MK-801 binds to and blocks the rat and human dopamine transporters. However, H3R activation had no effect on the inhibitory action of MK-801 on dopamine uptake and release. In PFCx slices, MK-801 and the activation of H3Rs or dopamine D1 receptors (D1Rs) stimulated ERK-1/2 and Akt phosphorylation. The co-activation of D1Rs and H3Rs prevented ERK-1/2 and Akt phosphorylation, and H3R activation or D1R blockade prevented the effect of MK-801. In ex vivo experiments, the intracortical infusion of the D1R agonist SKF-81297 (37 ng/1 μl) or the H3R agonist RAMH increased Akt phosphorylation, prevented by D1R/H3R co-activation. These results indicate that MK-801 enhances dopaminergic transmission in the PFCx, and that H3R activation counteracts the post-synaptic actions of dopamine.