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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., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 International Brain Research Organization (IBRO). Published by Elsevier Inc. All rights reserved.)

The zebrafish, Danio rerio, is a widely adopted in vivo model that conserves organs such as the liver, kidney, stomach, and brain, being, therefore, suitable for studying human diseases, drug discovery and toxicology. The brain aminergic systems are also conserved and the histamine H 1 , H 2 and H 3 receptors were previously cloned and identified in the zebrafish brain. Genome studies identified another putative H 2 receptor (Hrh2) with ∼50% sequence identity with H 2 receptor orthologs. In this study, we recombinantly expressed both zebrafish H 2 receptor paralogs (hrh2a and hrh2b) and compared their pharmacology with the human H 2 receptor ortholog. Our results showed that both zebrafish receptors conserve all the class A GPCR motifs. However, in contrast with the Hrh2a paralog, the Hrh2b does not possess all the amino acid residues shown to participate in histamine binding. The zebrafish Hrh2a receptor displays high affinity for [ 3 H]-tiotidine with a binding profile for H 2 receptor ligands similar to that of the human H 2 receptor. The zebrafish Hrh2a receptor couples to Gα S and Gα q/11 proteins, resulting in cAMP accumulation and activation of several reporter genes linked to the Gα q/11 pathway. Additionally, this receptor shows high constitutive activity, with histamine potency in the low nanomolar range for cAMP accumulation and the micromolar range for the activation of the NFAT response element. Moreover, dimaprit and amthamine seem to preferentially activate Gα S over Gα q/11 proteins via the zebrafish Hrh2a receptor. These results can contribute to clarifying the functional roles of the H 2 receptor in zebrafish., Competing Interests: Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Daniel A. McNaught-Flores reports financial support was provided by National Council on Science and Technology (Mexico). Rob Leurs reports financial support was provided by Dutch Research Council. Pertti Panula reports financial support was provided by Jane and Aatos Erkko Foundation. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Authors. Published by Elsevier B.V. All rights reserved.)

This study attempted to clarify the role of histamine H 1 receptors in epilepsy by exploring the effects of agonists and inverse agonists on the rundown of the current induced by iterative applications of NMDA or GABA in primary neuronal culture. Mepyramine, a classical H 1 -receptor antagonist/inverse agonist, increased the NMDA current by about 40% during the first minutes of recording. This effect was concentration-dependent, maximal at 10 nM, and mimicked by triprolidine, another antagonist/inverse agonist. No endogenous histamine was detected in the cultures by a selective immunoassay; both compounds were acting as inverse agonists. Indicating a high constitutive activity of the H 1 receptor in this system, histamine did not affect the NMDA rundown, including its settlement, but significantly reversed the effect of mepyramine. A similar pattern was obtained with 2,3 bromophenyl histamine, a selective H 1 -receptor agonist. The initial increase induced by the two inverse agonists was followed by the same rundown as in controls. H 1 - and NMDA receptors are colocalized in most cultured neuronal cells. Mepyramine and histamine did not affect the GABA rundown. Our findings suggest an interaction between H 1 - and NMDA receptors. Inactivation of the H 1 -receptor by its inverse agonists delays the settlement of the NMDA rundown, which may underlie their proconvulsant effect reported in clinics. Therefore, H 1 -receptor constitutive activity and the effect of histamine revealed in its absence, tend to facilitate the initiation of the rundown, which is consistent with the anticonvulsant properties of histamine via activation of H 1 -receptors reported in many studies., (© 2024 The Author(s). Pharmacology Research & Perspectives published by British Pharmacological Society and American Society for Pharmacology and Experimental Therapeutics and John Wiley & Sons Ltd.)

The histamine H 3 receptor (H 3 R) is a neurotransmitter receptor that is primarily found in the brain, where it controls the release and synthesis of histamine, as well as the release of other neurotransmitters (e.g. dopamine, serotonin). Notably, 20 H 3 R isoforms are differentially expressed in the human brain as a consequence of alternative gene splicing. The hH 3 R-445, -415, -365 and -329 isoforms contain the prototypical GPCR (7TM) structure, yet exhibit deletions in the third intracellular loop, a structural domain that is pivotal for G protein-coupling, signaling and regulation. To date, the physiological relevance underlying the individual and combinatorial function of hH 3 R isoforms remains poorly understood. Nevertheless, given their significant implication in physiological processes (e.g. cognition, homeostasis) and neurological disorders (e.g. Alzheimer's and Parkinson's disease, schizophrenia), widespread targeting of hH 3 R isoforms by drugs may lead to on-target side effects in brain regions that are unaffected by disease. To this end, isoform- and/or pathway-selective targeting of hH 3 R isoforms by biased agonists could be of therapeutic relevance for the development of region- and disease-specific drugs. Hence, we have evaluated ligand biased signaling at the hH 3 R-445, -415, -365 and -329 isoforms across various Gα i/o -mediated (i.e. [ 35 S]GTPγS accumulation, cAMP inhibition, pERK1/2 activation, pAKT T308/S473 activation) and non Gα i/o -mediated (i.e. β-arrestin2 recruitment) endpoints that are relevant to neurological diseases. Our findings indicate that H 3 R agonists display significantly altered patterns in their degree of ligand bias, in a pathway- and isoform-dependent manner, underlining the significance to investigate GPCRs with multiple isoforms to improve development of selective drugs. SUBJECT CATEGORY: Neuropharmacology., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 The Author(s). Published by Elsevier Inc. All rights reserved.)

Histamine is a biogenic amine that is critical in various physiological and pathophysiological processes, including but not limited to allergic reactions, wakefulness, gastric acid secretion and neurotransmission. Here, we determine 9 cryo-electron microscopy (cryo-EM) structures of the 4 histamine receptors in complex with four different G protein subtypes, with endogenous or synthetic agonists bound. Inside the ligand pocket, we identify key motifs for the recognition of histamine, the distinct binding orientations of histamine and three subpockets that facilitate the design of specific ligands. In addition, we also identify key residues responsible for the selectivity of immethridine. Moreover, we reveal distinct structural features as determinants of Gq vs. Gs or Gs vs. Gi coupling differences among the histamine receptors. Our study provides a structural framework for understanding the ligand recognition and G protein coupling of all 4 histamine receptors, which may facilitate the rational design of ligands targeting these receptors., (© 2024. The Author(s).)

Diazepam administration has been shown to influence the release of histamine in various brain areas involved in motor behavior. Therefore, the present study explored the plausible regulatory role of the central histaminergic system in diazepam-induced deficits in motor performance in mice using the rota-rod and beam walking tests. In this study, several doses of diazepam (0.5, 1, 2, and 3 mg/kg, i.p.) were assessed in mice for changes in motor performance on the rota-rod and beam walking test. In addition, the brain histamine levels were determined after diazepam administration, and the diazepam-induced motor deficits were assessed in mice, pretreated centrally (intracerebroventricular) with histaminergic agents such as histamine (0.1, 10 µg), histamine precursor (L-histidine: 0.1, 2.5 µg), histamine neuronal releaser/H 3 receptor antagonist (thioperamide: 0.5, 10 µg), H 1 and H 2 receptor agonist [2-(3-trifluoromethylphenyl) histamine (FMPH: 0.1, 6.5 µg; amthamine: 0.1, 5 µg)/antagonist (H 1 : cetirizine 0.1 µg) and (H 2 : ranitidine: 50 µg)]. Results indicate that mice treated with diazepam at doses 1, 2 mg/kg, i.p. significantly increased the brain histamine levels. Moreover, in mice pretreated with histaminergic transmission-enhancing agents, the diazepam (2 mg/kg, i.p.)-induced motor incoordination was significantly reversed. Contrastingly, diazepam (1 mg/kg, i.p.) in its subeffective dose produced significant motor deficits in mice preintracerebroventricular injected with histamine H 1 and H 2 receptor antagonists on both the employed tests. Therefore, it is postulated that endogenous histamine operates via H 1 and H 2 receptor activation to alleviate the motor-impairing effects of diazepam., (Copyright © 2024 Wolters Kluwer Health, Inc. All rights reserved.)

Prenatal alcohol exposure can have persistent effects on learning, memory, and synaptic plasticity. Previous work from our group demonstrated deficits in long-term potentiation (LTP) of excitatory synapses on dentate gyrus granule cells in adult offspring of rat dams that consumed moderate levels of alcohol during pregnancy. At present, there are no pharmacotherapeutic agents approved for these deficits. Prior work established that systemic administration of the histaminergic H3R inverse agonist ABT-239 reversed deficits in LTP observed following moderate PAE. The present study examines the effect of a second H3R inverse agonist, SAR-152954, on LTP deficits following moderate PAE. We demonstrate that systemic administration of 1 mg/kg of SAR-152954 reverses deficits in potentiation of field excitatory post-synaptic potentials (fEPSPs) in adult male rats exposed to moderate PAE. Time-frequency analyses of evoked responses revealed PAE-related reductions in power during the fEPSP, and increased power during later components of evoked responses which are associated with feedback circuitry that are typically not assessed with traditional amplitude-based measures. Both effects were reversed by SAR-152954. These findings provide further evidence that H3R inverse agonism is a potential therapeutic strategy to address deficits in synaptic plasticity associated with PAE., Competing Interests: Declaration of competing interest The authors declare no conflicts of interest., (Copyright © 2024 Elsevier Inc. All rights reserved.)

The histamine H 3 receptor (H 3 R) regulates as a presynaptic G protein-coupled receptor the release of histamine and other neurotransmitters in the brain, and is consequently a potential therapeutic target for neuronal disorders. The human H 3 R encodes for seven splice variants that vary in the length of intracellular loop 3 and/or the C-terminal tail but are all able to induce heterotrimeric G i protein signaling. The last two decades H 3 R drug discovery and lead optimization has been exclusively focused on the 445 amino acids-long reference isoform H 3 R-445. In this study, we pharmacologically characterized for the first time all seven H 3 R isoforms by determining their binding affinities for reference histamine H 3 receptor agonists and inverse agonists. The H 3 R-453, H 3 R-415, and H 3 R-413 isoforms display similar binding affinities for all ligands as the H 3 R-445. However, increased agonist binding affinities were observed for the three shorter isoforms H 3 R-329, H 3 R-365, and H 3 R-373, whereas inverse agonists such as the approved anti-narcolepsy drug pitolisant (Wakix®) displayed significantly decreased binding affinities for the latter two isoforms. This opposite change in binding affinity of agonist versus inverse agonists on H 3 R-365 and H 3 R-373 is associated with their higher constitutive activity in a cAMP biosensor assay as compared to the other five isoforms. The observed differences in pharmacology between longer and shorter H 3 R isoforms should be considered in future drug discovery programs., Competing Interests: Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Meichun Gao reports financial support was provided by CSC Chinese scholarship grant. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Authors. Published by Elsevier B.V. All rights reserved.)

Steroid-based histamine H 3 receptor antagonists (d-homoazasteroids) were designed by combining distinct structural elements of HTS hit molecules. They were characterized, and several of them displayed remarkably high affinity for H 3 receptors with antagonist/inverse agonist features. Especially, the 17a-aza-d-homolactam chemotype demonstrated excellent H 3 R activity together with significant in vivo H 3 antagonism. Optimization of the chemotype was initiated with special emphasis on the elimination of the hERG and muscarinic affinity. Additionally, ligand-based SAR considerations and molecular docking studies were performed to predict binding modes of the molecules. The most promising compounds ( XXI , XXVIII , and XX ) showed practically no muscarinic and hERG affinity. They showed antagonist/inverse agonist property in the in vitro functional tests that was apparent in the rat in vivo dipsogenia test. They were considerably stable in human and rat liver microsomes and provided significant in vivo potency in the place recognition and novel object recognition cognitive paradigms.

The histamine H 3 receptor, prominently expressed in neurons with a minor presence in glial cells, acts as both an autoreceptor and an alloreceptor, controlling the release of histamine and other neurotransmitters. The receptor impacts various essential physiological processes. Our team's initial investigations had demonstrated that the histamine H 3 receptor antagonists could facilitate nerve regeneration by promoting the histamine H 1 receptors on primary neural stem cells (NSCs) in the traumatic brain injury mouse, which suggested the potential of histamine H 3 receptor as a promising target for treating neurological disorders and promoting nerve regeneration. Pitolisant (PITO) is the only histamine H 3 receptor antagonist approved by the Food and Drug Administration (FDA) for treating narcolepsy. However, there is no report on Pitolisant in neural development or regeneration, and it is urgent to be further studied in strong biological activity models in vitro. The embryonic stem (ES) cells were differentiated into neural cells in vitro, which replicated the neurodevelopmental processes that occur in vivo. It also provided an alternative model for studying neurodevelopmental processes and testing drugs for neurological conditions. Therefore, we aimed to elucidate the regulatory role of Pitolisant in the early differentiation of ES cells into neural cells. Our results demonstrated that Pitolisant could promote the differentiation of ES cells toward NSCs and stimulated the formation of growth cones. Furthermore, Pitolisant was capable of inducing the polarization of NSCs through the cAMP-LKB1-SAD/MARK2 pathway, but had no significant effect on later neuronal maturation. Pitolisant altered mitochondrial morphology and upregulated the levels of mitochondrion-related proteins TOM20, Drp1, and p-Drp1, and reversed the inhibitory effect of Mdivi-1 on mitochondrial fission during the early neural differentiation of ES cells. In addition, Pitolisant induced the increase in cytosolic Ca 2+ . Our study provided an experimental foundation for the potential application of histamine H 3 receptor-targeted modulators in the field of neuroregeneration.

Many structures of the central nervous system recruit different neurotransmitters in pain processing. This study focused on the contribution of histamine and its H 1 receptors in the ventral pallidum (VP) in mediating pain-triggered behaviors. Intra-VP microinjection of histamine and 2-pyridylethylamine (2-PEA, a histamine H 1 receptor agonist) at the same doses of 0.5 and 1 µg/200 nl reduced both the first and second phases of licking/biting duration as well as flinching number induced by intra-plantar (ipl) injection of formalin (2.5%, 50 µl). Premicroinjection of mepyramine (a histamine H 1 antagonist, 2 µg/200 nl) into the VP antagonized the suppressive effects of 1 µg/200 nl histamine and 2-PEA on licking/biting and flinching behaviors. The possible mechanisms of the above-mentioned pain-reducing effects were followed by intra-VP and intrathecal administration of naloxone (an opioid receptor antagonist). Naloxone (2 µg/200 nl) preadministration into the VP inhibited attenuating effects of histamine and 2-PEA on both the licking/biting and flinching behaviors, whereas intrathecal injection of naloxone only inhibited their suppressing effects on flinching behavior. None of the treatments used in this study altered the animal's motor activity. The obtained results may reveal the role of histamine and its activated H 1 receptor in the VP in suppressing the pain behaviors caused by formalin. Opioid receptors in the VP and spinal cord may contribute to these functions., (Copyright © 2023 Wolters Kluwer Health, Inc. All rights reserved.)

Histamine is a biogenic amine that participates in allergic and inflammatory processes by stimulating histamine receptors. The histamine H 4 receptor (H 4 R) is a potential therapeutic target for chronic inflammatory diseases such as asthma and atopic dermatitis. Here, we show the cryo-electron microscopy structures of the H 4 R-G q complex bound with an endogenous agonist histamine or the selective agonist imetit bound in the orthosteric binding pocket. The structures demonstrate binding mode of histamine agonists and that the subtype-selective agonist binding causes conformational changes in Phe344 7.39 , which, in turn, form the "aromatic slot". The results provide insights into the molecular underpinnings of the agonism of H 4 R and subtype selectivity of histamine receptors, and show that the H 4 R structures may be valuable in rational drug design of drugs targeting the H 4 R., (© 2023. Springer Nature Limited.)

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-G q 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-G q 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-G q complex and rather induces an inactive conformation in the absence of the G q 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.

Pitolisant is a histamine 3-receptor antagonist/inverse agonist effective and safe for the treatment of excessive daytime sleepiness and cataplexy in narcolepsy. We report a 19-year-old woman affected by narcolepsy type 1 who presented panic attacks and dissociative symptoms induced by pitolisant. The patient medical history was unremarkable except that for familiarity for anxiety disorder and chronic insomnia. Moreover, a detailed psychometric evaluation revealed a profile of low resilience, a severe grade of depression, an anxiety trait and a propension to dissociative symptoms. Our report suggests that caution should be used in patients with predisposing factors to psychiatric disorders, especially during the first period of treatment with pitolisant. In consideration of the high prevalence of psychiatric comorbidities in narcolepsy, it seems worth to carefully investigate psychiatric background of narcoleptic patients., (Copyright © 2022 Wolters Kluwer Health, Inc. All rights reserved.)

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

Histamine H 3 receptor (H 3 R) agonists without an imidazole moiety remain very scarce. Of these, ZEL-H16 (1) has been reported previously as a high-affinity non-imidazole H 3 R (partial) agonist. Our structure-activity relationship analysis using derivatives of 1 identified both basic moieties as key interaction motifs and the distance of these from the central core as a determinant for H 3 R affinity. However, in spite of the reported H 3 R (partial) agonism, in our hands, 1 acts as an inverse agonist for Gα i signaling in a CRE-luciferase reporter gene assay and using an H 3 R conformational sensor. Inverse agonism was also observed for all of the synthesized derivatives of 1. Docking studies and molecular dynamics simulations suggest ionic interactions/hydrogen bonds to H 3 R residues D114 3.32 and E206 5.46 as essential interaction points., (© 2022 The Authors. Archiv der Pharmazie published by Wiley-VCH GmbH on behalf of Deutsche Pharmazeutische Gesellschaft.)

Military personnel are often victims of spinal cord injury resulting in lifetime disability and decrease in quality of life. However, no suitable therapeutic measures are still available to restore functional disability or arresting the pathophysiological progression of disease in victims for leading a better quality of life. Thus, further research in spinal cord injury using novel strategies or combination of available neuroprotective drugs is urgently needed for superior neuroprotection. In this regard, our laboratory is engaged in developing TiO 2 nanowired delivery of drugs, antibodies and enzymes in combination to attenuate spinal cord injury induced pathophysiology and functional disability in experimental rodent model. Previous observations show that histamine antagonists or antioxidant compounds when given alone in spinal cord injury are able to induce neuroprotection for short periods after trauma. In this investigation we used a combination of histaminergic drugs with antioxidant compound H-290/51 using their nanowired delivery for neuroprotection in spinal cord injury of longer duration. Our observations show that a combination of H3 receptor inverse agonist BF-2549 with H3 receptor antagonist and H4 receptor agonist clobenpropit induced neuroprotection is potentiated by antioxidant compound H-290/51 in spinal cord injury. These observations suggests that histamine receptors are involved in the pathophysiology of spinal cord injury and induce superior neuroprotection in combination with an inhibitor of lipid peroxidation H-290/51, not reported earlier. The possible mechanisms and significance of our findings in relation to future clinical approaches in spinal cord injury is discussed., Competing Interests: Conflict of interest There is no conflict of interest between any entity and/or organization mentioned here., (Copyright © 2023. Published by Elsevier Inc.)

The research was performed to study the mechanism whereby histamine affects the profile of plasma lipids. Six groups of ten male rats were received two injections with histamine or its H1- and H2-agonists and antagonists. Histamine caused a significant decrease in the concentrations of triglyceride, total cholesterol, and LDLc, while HDLc had no significant change. The rate of VLDL secretion was 263.6 ± 25.8 mg/h dL in control rats and was inhibited by about 68% in histamine injected rats. These changes have been mimicked by either histamine H1- or H2-agonists. The effects of H1- and H2-agonists were abolished in the presence of cetirizine and famotidine respectively. Histamine causes a significant decrease in serum triglyceride, total, and LDL-cholesterol by both H1 and H2-receptors. The decrease in serum lipids is due to the inhibitory effect of histamine or its agonists on VLDL secretion from the liver.

Histamine is well known for mediating peripheral inflammation; however, this amine is also found in high concentrations in the brain where its roles are much less known. In vivo chemical dynamics are difficult to measure, thus fundamental aspects of histamine's neurochemistry remain undefined. In this work, we undertake the first in-depth characterization of real time in vivo histamine dynamics using fast electrochemical tools. We find that histamine release is sensitive to pharmacological manipulation at the level of synthesis, packaging, autoreceptors and metabolism. We find two breakthrough aspects of histamine modulation. First, differences in H3 receptor regulation between sexes show that histamine release in female mice is much more tightly regulated than in male mice under H3 or inflammatory drug challenge. We hypothesize that this finding may contribute to hormone-mediated neuroprotection mechanisms in female mice. Second, a high dose of a commonly available antihistamine, the H1 receptor inverse agonist diphenhydramine, rapidly decreases serotonin levels. This finding highlights the sheer significance of pharmaceuticals on neuromodulation. Our study opens the path to better understanding and treating histamine related disorders of the brain (such as neuroinflammation), emphasizing that sex and modulation (of serotonin) are critical factors to consider when studying/designing new histamine targeting therapeutics.

Histamine is a major neurotransmitter and alleviates neuronal damage after ischemic injury via H 2 receptors. Herein, we investigated the effects of H 2 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. H 2 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 H 2 receptor antagonist, inhibited these effects. Expression of the H 2 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 H 2 receptor agonists alleviate TBI-induced BBB disruption by increasing the expression of vascular protective factors in astrocytes and BMECs., Competing Interests: Declaration of competing interest The authors declare no conflict of interest., (Copyright © 2022 The Authors. Production and hosting by Elsevier B.V. All rights reserved.)

Studies have shown that brain histamine has a role in seizure pathophysiology. Histamine acts by four distinct receptor subtypes (H1R-H4R). Previous reports signified the anticonvulsant activity of histamine H3R antagonists. We evaluated the effect of intra-amygdala injection of pitolisant the H3R inverse agonist on seizures induced by the electrical kindling model of epilepsy. Eighteen adult male rats with an approximate weight of 300 g were used. A tri-polar electrode twisted with the guide cannula, and two monopolar electrodes were implanted into the basolateral amygdala or the surface of the skull using stereotaxic surgery. One week after surgery, the threshold was determined in the animals. Twenty-four hours afterward, the animals received six stimuli daily with the threshold intensity until the generation of three consecutive stages five seizures. Then, saline, and 24 h later, pitolisant at three doses (1, 10, and 100 μg) were injected into the amygdala in distinct rats. Thirty minutes after injection of the drug or its solvent, seizure parameters including after-discharge duration (ADD), seizure stage (SS), and stage five duration (S5D) were recorded. Data analysis indicated that pitolisant reduced S5D at all doses, significantly. Pitolisant at the dose of 100 µg also decreased ADD and SS, significantly. However, pitolisant at the doses of 1 and 10 µg did not change ADD and SS. The dose-response curves showed that the anticonvulsant activity of pitolisant changed in a dose-dependent manner. In conclusion, the results confirmed the powerful anticonvulsant effects of pitolisant in the electrical kindling model of epilepsy., (Copyright © 2022 Elsevier B.V. All rights reserved.)

Histamine is a neurotransmitter that modulates neuronal activity and regulates various brain functions. Histamine H 3 receptor (H 3 R) antagonists/inverse agonists enhance its release in most brain regions, including the cerebral cortex, which improves learning and memory and exerts an antiepileptic effect. However, the mechanism underlying the effect of H 3 R antagonists/inverse agonists on cortical neuronal activity in vivo remains unclear. Here, we show the mechanism by which pitolisant, an H 3 R antagonist/inverse agonist, influenced perirhinal cortex (PRh) activity in individual neuron and neuronal population levels. We monitored neuronal activity in the PRh of freely moving mice using in vivo Ca 2+ imaging through a miniaturized one-photon microscope. Pitolisant increased the activity of some PRh neurons while decreasing the activity of others without affecting the mean neuronal activity across neurons. Moreover, it increases neuron pairs with synchronous activity in excitatory-responsive neuronal populations. Furthermore, machine learning analysis revealed that pitolisant altered the neuronal population activity. The changes in the population activity were dependent on the neurons that were excited and inhibited by pitolisant treatment. These findings indicate that pitolisant influences the activity of a subset of PRh neurons by increasing the synchronous activity and modifying the population activity., (© 2022. The Author(s).)

Histamine has been implicated in urinary bladder dysfunction as an inflammatory mediator driving sensory nerve hypersensitivity. However, the direct influence of histamine on smooth muscle has not been thoroughly investigated. We hypothesized that histamine directly contracts urinary bladder smooth muscle (UBSM) independent of effects on nerves. Single cell quantitative RT-PCR determined that only histamine H 1 and H 2 receptors were expressed on UBSM cells. In isolated tissue bath experiments, histamine (200 µM) caused a highly variable and rapidly desensitizing contraction that was completely abolished by the H 1 receptor antagonist fexofenadine (5 µM) and the G q/11 inhibitor YM254890 (1 µM). Neither the muscarinic receptor antagonist atropine (1 µM), the Na + channel blocker tetrodotoxin (1 µM), nor the transient receptor potential vanilloid type 1 antagonist capsazepine (10 µM) altered responses to histamine, suggesting that nerve activation was not involved. UBSM desensitization to histamine was not due to receptor internalization, as neither the cholesterol-depleting agent methyl-β-cyclodextrin (10 mM), the dynamin-mediated endocytosis inhibitor dynasore (100 µM), nor the clathrin-mediated endocytosis inhibitor pitstop2 (15 µM) augmented or prolonged histamine contractions. Buffer from desensitized tissues still contracted histamine-naïve tissues, revealing that histamine was not metabolized. Prolonged exposure to histamine also had no effect on contractions due to electrical field stimulation, suggesting that both efferent nerve and UBSM excitability were unchanged. Together, these data suggest that histamine, although able to transiently contract UBSM, does not have a lasting effect on UBSM excitability or responses to efferent nerve input. Thus, any acute effects of histamine directly on UBSM contractility are unlikely to alter urinary bladder function. NEW & NOTEWORTHY Histamine is commonly associated with inflammatory bladder pathologies. We sought to investigate the role of histamine on urinary bladder contractility. Histamine contracts the bladder, but this response is highly variable and desensitizes completely in minutes. This desensitization is not due to internalization of the receptor or metabolism of histamine. Because nerve-evoked contractions are also not increased in the presence of histamine, our findings suggest that histamine is not directly acting to change contractility.

Several of the drugs currently available for the treatment of premature ejaculation (PE) (e.g., local anesthetics or antidepressants) are associated with numerous safety concerns and exhibit weak efficacy. To date, no therapeutics for PE have been approved in the United States, highlighting the need to develop novel agents with sufficient efficacy and fewer side effects. In this study, we focused on the histamine H 3 receptor (H 3 R) as a potential target for the treatment of PE and evaluated the effects of imetit (an H 3 R/H 4 R agonist), ciproxifan (an H 3 R antagonist), and JNJ-7777120 (an H 4 R antagonist) in vivo. Our in vivo electrophysiological experiments revealed that imetit reduced mechanical stimuli-evoked neuronal firing in anesthetized rats. This effect was inhibited by ciproxifan but not by JNJ-7777120. Subsequently, we evaluated the effect of imetit using a copulatory behavior test to assess ejaculation latency (EL) in rats. Imetit prolonged EL, although this effect was inhibited by ciproxifan. These findings indicate that H 3 R stimulation suppresses mechanical stimuli-evoked neuronal firing in the spinal-penile neurotransmission system, thereby resulting in prolonged EL. To our knowledge, this is the first report to describe the relationship between H 3 R and PE. Thus, H 3 R agonists may represent a novel treatment option for PE.

A series of chemical optimizations, which was guided by in vitro affinity at histamine H 3 receptor (H 3 R), modulation of lipophilicity, ADME properties and preclinical efficacy resulted in the identification of 1-[2-(1-cyclobutylpiperidin-4-yloxy)-6,7-dihydro-4H-thiazolo[5,4-c]pyridin-5-yl]propan-1-one (45 e) as a potent and selective (K i =4.0 nM) H 3 R inverse agonist. Dipsogenia induced by (R)-α-methylhistamine was dose dependently antagonized by 45 e, confirming its functional antagonism at H 3 R. It is devoid of hERG and phospholipidosis issues. Compound 45 e has adequate oral exposures and favorable half-life in both rats and dogs. It has demonstrated high receptor occupancy (ED 80 =0.22 mg/kg) and robust efficacy in object recognition task and, dose dependently increased acetylcholine levels in brain. The sub-therapeutic doses of 45 e in combination with donepezil significantly increased acetylcholine levels. The potent affinity, selectivity, in vivo efficacy and drug like properties together with safety, warrant for further development of this molecule for potential treatment of cognitive disorders associated with Alzheimer's disease., (© 2021 Wiley-VCH GmbH.)

Unmyelinated tactile (C-tactile or CT) afferents are abundant in arm hairy skin and have been suggested to signal features of social affective touch. Here, we recorded from unmyelinated low-threshold mechanosensitive afferents in the peroneal and radial nerves. The most distal receptive fields were located on the proximal phalanx of the third finger for the superficial branch of the radial nerve and near the lateral malleolus for the peroneal nerve. We found that the physiological properties with regard to conduction velocity and mechanical threshold, as well as their tuning to brush velocity, were similar in CT units across the antebrachial ( n = 27), radial ( n = 8), and peroneal ( n = 4) nerves. Moreover, we found that although CT afferents are readily found during microneurography of the arm nerves, they appear to be much more sparse in the lower leg compared with C-nociceptors. We continued to explore CT afferents with regard to their chemical sensitivity and found that they could not be activated by topical application to their receptive field of either the cooling agent menthol or the pruritogen histamine. In light of previous studies showing the combined effects that temperature and mechanical stimuli have on these neurons, these findings add to the growing body of research suggesting that CT afferents constitute a unique class of sensory afferents with highly specialized mechanisms for transducing gentle touch. NEW & NOTEWORHY Unmyelinated tactile (CT) afferents are abundant in arm hairy skin and are thought to signal features of social affective touch. We show that CTs are also present but are relatively sparse in the lower leg compared with C-nociceptors. CTs display similar physiological properties across the arm and leg nerves. Furthermore, CT afferents do not respond to the cooling agent menthol or the pruritogen histamine, and their mechanical response properties are not altered by these chemicals.

The discovery of the human histamine H4 receptor (H4R) has contributed to our understanding of the role of histamine in numerous physiological and pathological conditions, including tumor development and progression. The lymph nodes of patients with malignant lymphomas have shown to contain high levels of histamine, however, less is known regarding the expression and function of the H4R in T-cell lymphoma (TCL). In this work we demonstrate the expression of H4R isoforms (mRNA and protein) in three human aggressive TCL (OCI-Ly12, Karpas 299, and HuT78). Histamine and specific H4R agonists (VUF8430 and JNJ28610244) significantly reduced cell viability in a dose-dependent manner ( p < 0.05). The combined treatment with the H4R antagonist (JNJ7777120, 10 µM) reversed the effects of the H4R ligands. Importantly, we screened a drug repurposing library of 433 FDA-approved compounds (1 μM) in combination with histamine (10 μM) in Hut78 cells. Histamine produced a favorable antitumor effect with 18 of these compounds, including the histone deacetylase inhibitor panobinostat. Apoptosis, proliferation, and oxidative stress studies confirmed the antitumoral effects of the combination. We conclude that the H4R is expressed in TCL, and it is involved in histamine-mediated responses.

Background: Autism spectrum disorders (ASDs) are a group of neurodevelopmental conditions defined by behavioral deficits in social communication and interactions, mental inflexibility and repetitive behaviors. Converging evidence from observational and preclinical studies suggest that excessive repetitive behaviors in people with ASD may be due to elevated histaminergic H3 receptor signaling in the striatum. We hypothesized that systemic administration of pharmacological histamine H3 receptor antagonists would attenuate the expression of repetitive behaviors in the BTBR T+Itpr3tf/J (BTBR) mouse inbred strain, an established mouse model presenting autism-like repetitive behaviors and novelty-induced hyperactivity. We further aimed to investigate whether agonism of the histamine H3 receptor would be sufficient to induce repetitive behaviors in the C57BL/6J control mouse strain., Methods: Different doses of H3 receptor agonists (i.e., (R)-α-methylhistamine and immethridine) and H3 receptor antagonists/inverse agonists (i.e., ciproxifan and pitolisant) were administered via intraperitoneal (i.p.) injection in male mice to characterize the acute effects of these compounds on ASD-related behavioral readouts., Results: The highly selective H3 receptor agonist immethridine significantly increased the time spent in stereotypic patterns in C57BL/6J mice, but this effect appeared to be driven by general sedative properties of the compound. High doses of pitolisant significantly decreased locomotor hyperactivity in novel environments in BTBR mice, without significant effects on repetitive behaviors., Conclusions: Based on our findings, we conclude that acute H3 receptor manipulation mainly affected general motor activity levels in novel environments. Small changes in stereotyped behaviors were observed but appeared to be driven by altered general activity levels., (Copyright © 2021 The Authors. Published by Elsevier Inc. All rights reserved.)

In pharmacological studies conducted on animals over the last four decades, histamine was determined to be a strong modulator of learning and memory. Activation of histamine signaling enhances memory consolidation and retrieval. Even long after learning and forgetting, it can still restore the retrieval of forgotten memories. These findings based on animal studies led to human clinical trials with histamine H 3 receptor antagonists/inverse agonists, which revealed their positive effects on learning and memory. Therefore, histamine signaling is a promising therapeutic target for improving cognitive impairments in patients with various neuropsychiatric disorders, including Alzheimer's disease. While the memory-modulatory effects of histamine receptor agonists and antagonists have been confirmed by several research groups, the underlying mechanisms remain to be elucidated. This review summarizes how the activation and inhibition of histamine signaling influence memory processes, introduces the cellular and circuit mechanisms, and discusses the relationship between the human histaminergic system and learning and memory., (© 2021. The Author(s), under exclusive license to Springer Nature Switzerland AG.)

Carbonic anhydrases (CAs, EC 4.2.1.1) activators were shown to be involved in memory enhancement and learning in animal models of cognition. Here we investigated the CA activating effects of a large series of histamine based compounds, including histamine receptors (H 1 R - H 4 R) agonists, antagonists and other derivatives of this autacoid. CA activators may be thus useful for improving cognition as well as in diverse therapeutic areas (phobias, obsessive-compulsive disorder, generalised anxiety, post-traumatic stress disorders), for which activation of this enzyme was recently shown to be involved.

The responsiveness of the basilar artery to intrinsic vasoactive substances is species-specific and can be a unique characteristic. We investigated the responsiveness of the bat (Rhinolophus ferrumequinum) basilar artery to noradrenaline (NA), 5-hydroxytryptamine (5-HT), angiotensin (Ang) II, bradykinin (BK), histamine (His), and acetylcholine (ACh). NA, 5-HT, Ang II, and BK induced contraction, whereas His and ACh induced relaxation, in a concentration-dependent manner. The NA cumulative concentration-response curve was shifted to the right in parallel with phentolamine (an α-antagonist). However, propranolol, a β-antagonist, had no significant effect. The 5-HT curve was shifted to the right in parallel by ketanserin (a 5-HT 2 antagonist) and methiothepin (a 5-HT 1 and 5-HT 2 antagonist). Losartan (an AT 1 antagonist) shifted the Ang II curve to the right, whereas PD123319 (an AT 2 antagonist) had no significant effect. L-NA, indomethacin, and des-Arg 9 -[Leu 8 ]-BK (a B 1 antagonist) did not significantly affect BK-induced contractions. HOE140 (a B 2 antagonist) shifted the BK concentration-response curve to the right. The His curve was shifted to the right weakly by diphenhydramine (an H 1 antagonist) and strongly by cimetidine (a H 2 antagonist). ACh-induced relaxation was significantly inhibited by L-NA, atropine, and pFHHSiD (a muscarinic M 3 antagonist), whereas pirenzepine and methoctramine (muscarinic M 1 and M 2 antagonists, respectively) showed no significant effects. At a resting vascular tone, L-NA-induced contraction and indomethacin induced relaxation. These results suggest that α-adrenergic, 5-HT 1 , 5-HT 2 , AT 1 , and B 2 receptors might be important in arterial contraction, whereas M 3 and H 2 (>H 1 ) receptors might modify these contractions, inducing relaxation., (Copyright © 2021 Elsevier Inc. All rights reserved.)

During the past decades, histamine H 3 receptors have received widespread attention in pharmaceutical research due to their involvement in pathophysiology of several diseases such as neurodegenerative disorders. In this context, blocking of these receptors is of paramount importance in progression of such diseases. In the current investigation, novel histamine H 3 receptor ligands were designed by exploiting scaffold-hopping drug-design strategy. We inspected the designed molecules in terms of ADME properties, drug-likeness, as well as toxicity profiles. Additionally molecular docking and dynamics simulation studies were performed to predict binding mode and binding free energy calculations, respectively. Among the designed structures, we selected compound d2 and its demethylated derivative as examples for synthesis and affinity measurement. In vitro binding assays of the synthesized molecules demonstrated that d2 has lower binding affinity (K i  = 2.61 μM) in radioligand displacement assay to hH 3 R than that of demethylated form (K i  = 12.53 μM). The newly designed compounds avoid of any toxicity predictors resulted from extended in silico and experimental studies, can offer another scaffold for histamine H 3 R antagonists for further structure-activity relationship studies., (Copyright © 2021. Published by Elsevier Inc.)

So far, only little is known about the internalization process of the histamine H 2 receptor (H 2 R). One promising approach to study such dynamic processes is the use of agonistic fluorescent ligands. Therefore, a series of carbamoylguanidine-type H 2 R agonists containing various fluorophores, heterocycles, and linkers (28-40) was synthesized. The ligands were pharmacologically characterized in several binding and functional assays. These studies revealed a significantly biased efficacy (E max ) for some of the compounds, e.g. 32: whereas 32 acted as strong partial (E max : 0.77, mini-Gs recruitment) or full agonist (E max : 1.04, [ 35 S]GTPγS binding) with respect to G protein activation, it was only a weak partial agonist regarding β-arrestin1/2 recruitment (E max : 0.09-0.12) and failed to promote H 2 R internalization (confocal microscopy). On the other hand, H 2 R internalization was observed for compounds that exhibited moderate agonistic activity in the β-arrestin1/2 pathways (E max  ≥ 0.22). The presented differently-biased fluorescent ligands are versatile molecular tools for future H 2 R studies on receptor trafficking and internalization e.g. using fluorescence microscopy., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

In an integrative approach, we studied cardiac effects of recently published novel H 2 receptor agonists in the heart of mice that overexpress the human H 2 receptor (H 2 -TG mice) and littermate wild type (WT) control mice and in isolated electrically driven muscle preparations from patients undergoing cardiac surgery. Under our experimental conditions, the H 2 receptor agonists UR-Po563, UR-MB-158, and UR-MB-159 increased force of contraction in left atrium from H 2 -TG mice with pEC 50 values of 8.27, 9.38, and 8.28, respectively, but not in WT mice. Likewise, UR-Po563, UR-MB-158, and UR-MB-159 increased the beating rate in right atrium from H 2 -TG mice with pEC 50 values of 9.01, 9.24, and 7.91, respectively, but not from WT mice. These effects could be antagonized by famotidine, a H 2 receptor antagonist. UR-Po563 (1 µM) increased force of contraction in Langendorff-perfused hearts from H 2 -TG but not WT mice. Similarly, UR-Po563, UR-MB-158, or UR-MB-159 increased the left ventricular ejection fraction in echocardiography of H 2 -TG mice. Finally, UR-Po563 increased force of contraction in isolated human right atrial muscle strips. The contractile effects of UR-Po563 in H 2 -TG mice were accompanied by an increase in the phosphorylation state of phospholamban. In summary, we report here three recently developed agonists functionally stimulating human cardiac H 2 receptors in vitro and in vivo. We speculate that these compounds might be of some merit to treat neurologic disorders if their cardiac effects are blocked by concomitantly applied receptor antagonists that cannot pass through the blood-brain barrier or might be useful to treat congestive heart failure in patients. SIGNIFICANCE STATEMENT: Recently, a new generation of histamine H 2 receptor (H 2 R) agonists has been developed as possible treatment option for Alzheimer's disease. Here, possible cardiac (side) effects of these novel H 2 R agonists have been evaluated., (Copyright © 2021 by The American Society for Pharmacology and Experimental Therapeutics.)

Aims: One of the therapeutic approaches in the treatment of obesity is the use of histamine H 3 receptor ligands. Histamine plays a significant role in eating behavior because it causes a loss of appetite and is considered to be a satiety signal released during food intake., Material and Methods: Histamine ligands were selected based on the preliminary studies which included determination of intrinsic activity and selected pharmacokinetic parameters. Female Wistar rats were fed palatable feed for 28 days and simultaneously the tested compounds were administered intraperitoneally at a dose of 10 mg/kg b.w./day. Rats' weight was evaluated daily and calories intake was evaluated once per week. At the end of experiment insulin and glucose tolerance tests was performed. Plasma levels of cholesterol, triglycerides, leptin, insulin, glucose, C-peptide and CRP were also determined. In order to rule out false-positive results the influence of tested compounds on spontaneous activity of rats was monitored., Results: Animals fed palatable feed and treated with KSK-61 or KSK-63 compounds showed the slowest weight gain which was comparable to the one observed in control animals. Both compounds with the highest pharmacological activity have also similar pharmacokinetic properties with quite long half-life and high volume of distribution indicating that they can freely cross most biological barriers. Some compounds, especially KSK-63, compensated for metabolic disorders., Conclusion: The presented study proves that search among the active histamine H 3 receptor ligands for the new therapeutic agents to treat obesity is justified. Compounds KSK-61 and KSK-63 can be considered as the leading structures., (Copyright © 2021 The Authors. Published by Elsevier Masson SAS.. All rights reserved.)

It is important to search for a promising therapeutic target or small molecules that can control excessive eating since limiting the intake of foods, especially tasty ones, could be effective in the treatment or prevention of obesity. Some studies indicate betahistine as an unique drug having the ability to ameliorate, for example, antipsychotic-induced weight gain. This study aimed to determine whether repeated administration of betahistine (histamine H1R agonist and H3R antagonist) could be beneficial in reducing the intake of tasty foods or the body's response to overeating via mechanisms such as by influencing the levels of hormones involved in the regulation of food intake or the levels of selected metabolic parameters. Studies were performed in the excessive eating model in rats, which perfectly illustrates the harmful high-caloric intake from freely available tasty products rich in sugar and fat. Our results indicated that repeated administration of betahistine to rats caused lower gain of body mass compared to the control rats fed palatable feed. Interestingly, betahistine treatment increased the consumption of cheese, which is a source of histamine. Although betahistine did not prevent the development of metabolic disorders, such as reduced glucose tolerance, in test animals, it significantly increased the level of high-density lipoprotein cholesterol, which could certainly be considered beneficial. Further studies should be conducted to investigate the effect of repeated administration of betahistine on satiety, gastrointestinal disorders, and the preference for histamine-containing foods., (Copyright © 2021 The Authors. Published by Elsevier Masson SAS.. All rights reserved.)

A decreased H 1 receptor activity is observed in the anterior cingulate cortex (aCgCx) of depressed patients. The role of this abnormality in the development of depression-related processes is unstudied. We examined the influence of a decreased brain H 1 receptor activity on rat behavior in the sucrose preference test. The H 1 receptor deficit was simulated by injection of an H 1 antagonist into the aCgCx; also, two aCgCx projection areas, lateral and medial entorhinal cortices were examined. A blockade of H 1 -receptors in the aCgCx and lateral entorhinal cortex (LEntCx) significantly reduced sucrose preference. These findings suggest the existence of H 1 receptor-mediated aCgCx-LEntCx circuitry mechanism regulating anhedonic-like behavior in rats. The presented data suggest that H 1 receptor-mediated processes might be a therapeutic target in depressive disorders., (Copyright © 2021 Elsevier B.V. All rights reserved.)

3-(2-Amino-4-methylthiazol-5-yl)propyl-substituted carbamoylguanidines are potent, subtype-selective histamine H 2 receptor (H 2 R) agonists, but their applicability as pharmacological tools to elucidate the largely unknown H 2 R functions in the central nervous system (CNS) is compromised by their concomitant high affinity toward dopamine D 2 -like receptors (especially to the D 3 R). To improve the selectivity, a series of novel carbamoylguanidine-type ligands containing various heterocycles, spacers, and side residues were rationally designed, synthesized, and tested in binding and/or functional assays at H 1-4 and D 2long/3 receptors. This study revealed a couple of selective candidates (among others 31 and 47 ), and the most promising ones were screened at several off-target receptors, showing good selectivities. Docking studies suggest that the amino acid residues (3.28, 3.32, E2.49, E2.51, 5.42, and 7.35) are responsible for the different affinities at the H 2 - and D 2long/3 -receptors. These results provide a solid base for the exploration of the H 2 R functions in the brain in further studies.

Intravenous drug self-administration remains the 'gold standard' for assessing abuse liability. Failure of a drug to maintain self-administration might indicate the absence of positive reinforcing effects but might also indicate the presence of aversive effects. Sensitivity to aversive and punishing effects of drugs (as well as nondrug stimuli) might collectively determine the likelihood of use, abuse and relapse. Using a choice procedure, this study compared the effects of remifentanil (mu opioid receptor agonist; 0.001-0.01 mg/kg/infusion) and histamine (H1-4 receptor agonist; 0.32-3.2 mg/kg/infusion), alone and in mixtures, to test the hypothesis that remifentanil/histamine mixtures are less reinforcing compared with remifentanil alone and less punishing compared with histamine alone. Male Sprague-Dawley rats (n = 10) chose between an intravenous infusion + a pellet and a pellet alone. Rats were indifferent to saline, chose remifentanil + a pellet over a pellet alone, and chose a pellet alone over histamine + a pellet. The effects of remifentanil/histamine mixtures generally were different from the constituent doses of histamine alone but not from remifentanil alone. A mixture containing 3.2 mg/kg/infusion histamine and either 0.001 or 0.0032 mg/kg/infusion remifentanil was not different from saline but was different from the effects of the constituent dose, insofar as choice increased compared with 3.2 mg/kg/infusion histamine alone and decreased compared with 0.001 or 0.0032 mg/kg/infusion remifentanil alone. Reinforcing doses of remifentanil combined with punishing doses of histamine can yield mixtures that are neither preferred nor avoided, offering 'proof-of-principle' for using drug mixtures to avoid adverse effects of opioid receptor agonists., (Copyright © 2021 Wolters Kluwer Health, Inc. All rights reserved.)

Rationale: Samelisant (SUVN-G3031) is a potent and selective histamine H3 receptor (H3R) inverse agonist with good brain penetration and oral bioavailability., Objectives: Pharmacological and neurochemical characterisation to support the utility of Samelisant (SUVN-G3031) in the treatment of sleep-related disorders like narcolepsy., Methods: Samelisant (SUVN-G3031) was tested in rat brain microdialysis studies for evaluation of modulation in histamine, dopamine and norepinephrine. Sleep EEG studies were carried out in orexin knockout mice to study the effects of Samelisant (SUVN-G3031) on the sleep-wake cycle and cataplexy., Results: Samelisant (SUVN-G3031) has a similar binding affinity towards human (hH3R; K i = 8.7 nM) and rat (rH3R; K i = 9.8 nM) H3R indicating no inter-species differences. Samelisant (SUVN-G3031) displays inverse agonist activity and it exhibits very high selectivity towards H3R. Samelisant (SUVN-G3031) treatment in mice produced a dose-dependent increase in tele-methylhistamine levels indicating the activation of histaminergic neurotransmission. Apart from increasing the levels of histamine, Samelisant (SUVN-G3031) also modulates dopamine and norepinephrine levels in the cerebral cortex while it has no effects on dopamine levels in the striatum or nucleus accumbens. Treatment with Samelisant (SUVN-G3031; 10 and 30 mg/kg, p.o.) produced a significant increase in wakefulness with a concomitant decrease in NREM sleep in orexin knockout mice subjected to sleep EEG. Samelisant (SUVN-G3031) also produced a significant decrease in Direct REM sleep onset (DREM) episodes, demonstrating its anticataplectic effects in an animal model relevant to narcolepsy. Modulation in cortical levels of histamine, norepinephrine and dopamine provides the neurochemical basis for wake-promoting and anticataplectic effects observed in orexin knockout mice., Conclusions: Pre-clinical studies of Samelisant (SUVN-G3031) provide a strong support for utility in the treatment of sleep-related disorders related to EDS and is currently being evaluated in a phase 2 proof of concept study in the USA for the treatment of narcolepsy with and without cataplexy.

Background: Central histamine H3 receptors are a family of presynaptic auto and heteroreceptors. Blockade of the presynaptic H3 receptors activates the downstream pathway(s) involved in the processes of learning and memory, making it a potential therapeutic option for ameliorating cognitive dysfunction. Samelisant (SUVN-G3031) is a potent and selective inverse agonist at the H3 receptors., Aim: The aim of this research is to study the effects of Samelisant in diverse animal models of cognitive functions., Methods: The effects of Samelisant on cognitive functions were studied using social recognition, object recognition and Morris water maze tasks. Neurochemical and electrophysiological effects of Samelisant were monitored using microdialysis and electroencephalography techniques., Results: Samelisant showed procognitive effects in diverse animal models of cognition at doses ranging from 0.3 to 3 mg/kg, per os ( p.o .) (social recognition and object recognition task). Samelisant significantly increased the brain acetylcholine levels in the cortex at doses of 10 and 20 mg/kg, p.o . In the Morris water maze task, combined administration of suboptimal doses of Samelisant and donepezil resulted in procognitive effects significantly larger than the either treatment. Similarly, Samelisant significantly potentiated the effects of donepezil on pharmacodynamic biomarkers of cognition i.e. acetylcholine levels in brain and neuronal theta oscillations., Conclusion: Samelisant may have potential utility in the treatment of cognitive deficits associated with hypocholinergic state.

The ability of recognizing familiar conspecifics is essential for many forms of social interaction including reproduction, establishment of dominance hierarchies, and pair bond formation in monogamous species. Many hormones and neurotransmitters have been suggested to play key roles in social discrimination. Here we demonstrate that disruption or potentiation of histaminergic neurotransmission differentially affects short (STM) and long-term (LTM) social recognition memory. Impairments of LTM, but not STM, were observed in histamine-deprived animals, either chronically ( Hdc -/- mice lacking the histamine-synthesizing enzyme histidine decarboxylase) or acutely (mice treated with the HDC irreversible inhibitor α-fluoromethylhistidine). On the contrary, restriction of histamine release induced by stimulation of the H 3 R agonist (VUF16839) impaired both STM and LTM. H 3 R agonism-induced amnesic effect was prevented by pre-treatment with donepezil, an acetylcholinesterase inhibitor. The blockade of the H 3 R with ciproxifan, which in turn augmented histamine release, resulted in a procognitive effect. In keeping with this hypothesis, the procognitive effect of ciproxifan was absent in both Hdc -/- and αFMH-treated mice. Our results suggest that brain histamine is essential for the consolidation of LTM but not STM in the social recognition test. STM impairments observed after H 3 R stimulation are probably related to their function as heteroreceptors on cholinergic neurons.

Histamine H 1 receptor ligands used clinically as antiallergics rank among the most widely prescribed and over-the-counter drugs in the world. They exert the therapeutic actions by blocking the effects of histamine, due to null or negative efficacy towards Gα q -phospholipase C (PLC)-inositol triphosphates (IP 3 )-Ca 2+ and nuclear factor-kappa B cascades. However, there is no information regarding their ability to modulate other receptor responses. The aim of the present study was to investigate whether histamine H 1 receptor ligands could display positive efficacy concerning receptor desensitization, internalization, signaling through Gα q independent pathways or even transcriptional regulation of proinflammatory genes. While diphenhydramine, triprolidine and chlorpheniramine activate ERK1/2 (extracellular signal-regulated kinase 1/2) pathway in A549 cells, pre-treatment with chlorpheniramine or triprolidine completely desensitize histamine H 1 receptor mediated Ca 2+ response, and both diphenhydramine and triprolidine lead to receptor internalization. Unlike histamine, histamine H 1 receptor desensitization and internalization induced by antihistamines prove to be independent of G protein-coupled receptor kinase 2 (GRK2) phosphorylation. Also, unlike the reference agonist, the recovery of the number of cell-surface histamine H 1 receptors is a consequence of de novo synthesis. On the other hand, all of the ligands lack efficacy regarding cyclooxygenase-2 (COX-2) and interleukin-8 (IL-8) mRNA regulation. However, a prolonged exposure with each of the antihistamines impaires the increase in COX-2 and IL-8 mRNA levels induced by histamine, even after ligand removal. Altogether, these findings demonstrate the biased nature of histamine H 1 receptor ligands contributing to a more accurate classification, and providing evidence for a more rational and safe use of them., (Copyright © 2021 Elsevier B.V. All rights reserved.)

The present study investigated the plausible modulatory role of central histaminergic transmission on the expression of nicotine withdrawal induced anxiety and somatic behavior in mice. Abrupt cessation of chronic nicotine (2 mg/kg, i.p. × 3/day) treatment for 12 days to mice, expressed increased anxiety in light & dark test and total abstinence (somatic) score at 24 h post nicotine withdrawal time. The somatic signs includes a composite score of all behaviors such as grooming, rearing, jumping, body shakes, forelimb tremors, head shakes, abdominal constrictions, scratching, empty mouth chewing or teeth chattering, genital licking, tail licking. Mice exhibited higher expression to nicotine withdrawal induced anxiety in light & dark test at 24 h post-nicotine withdrawal time on pre-treatment centrally (i.c.v) with histaminergic agents like histamine (0.1, 50 μg/mouse), histamine H 3 receptor inverse agonist, thioperamide (2, 10 μg/mouse), histamine H 1 receptor agonist, FMPH (2, 6.5 μg/mouse) or H 2 receptor agonist amthamine (0.1, 0.5 μg/mouse) or intraperitoneally (i.p.) with histamine precursor, l-histidine (250, 500 mg/kg) as compared to control nicotine withdrawn animals. Furthermore, mice pre-treated with all these histaminergic agents except histamine H 1 receptor agonist, FMPH shows exacerbated expression to post-nicotine withdrawal induced total abstinence (somatic) score in mice. On the other hand, central injection of selective histamine H 1 receptor antagonist, cetirizine (0.1 μg/mouse, i.c.v.) or H 2 receptor antagonist, ranitidine (50 μg/mouse, i.c.v) to mice 10 min before 24 h post-nicotine withdrawal time completely alleviated the expression of nicotine withdrawal induced anxiety and somatic behavior. Thus, it can be contemplated that the blockade of central histamine H 1 or H 2 receptor during the nicotine withdrawal phase could be a novel approach to mitigate the nicotine withdrawal associated anxiety-like manifestations. Contribution of endogenous histamine via H 1 or H 2 receptor stimulation in the nicotine withdrawal induced anxiety and somatic behavior is proposed., (Copyright © 2020 Elsevier B.V. All rights reserved.)

The inflammatory reaction influences the deposition of collagen within wound granulation tissue. The aim of the present study is to determine whether histamine acting directly on myofibroblasts derived from wound granulation tissue may influence collagen deposition. It also identifies the histamine receptor involved in this process. The experiments were carried out on cells isolated from the granulation tissue of a wound model (a polypropylene net inserted subcutaneously to rats) or intact rat skin. Collagen content was measured following the addition of different concentrations of histamine and treatment with histamine receptor antagonists (ketotifen - H1 inhibitor, ranitidine - H2 inhibitor) and a histamine receptor H1 agonist (2-pyridylethylamine dihydrochloride).The cells were identified as myofibroblasts: alpha-smooth muscle actin, vimentin, and desmin positive in all experimental conditions. Histamine increased the collagen level within both cell cultures, i.e., those isolated from granulation tissue or intact skin. It did not, however, influence the expression of either the collagen type I or III genes within the cultured myofibroblasts. Histamine activity was reduced by ketotifen (the H1 receptor inhibitor) and increased by the H1 receptor agonist, as demonstrated by changes in the levels of collagen in the myofibroblast culture. Histamine increased collagen content within the cultures, acting directly on myofibroblasts via H1 receptor stimulation.

The cardiac plexus, which contains parasympathetic ganglia, plays an important role in regulating cardiac function. Histamine is known to excite intracardiac ganglion neurons, but the underlying mechanism is obscure. In the present study, therefore, the effect of histamine on rat intracardiac ganglion neurons was investigated using perforated patch-clamp recordings. Histamine depolarized acutely isolated neurons with a half-maximal effective concentration of 4.5 μM. This depolarization was markedly inhibited by the H 1 receptor antagonist triprolidine and mimicked by the H 1 receptor agonist 2-pyridylethylamine, thus implicating histamine H 1 receptors. Consistently, reverse transcription-PCR (RT-PCR) and Western blot analyses confirmed H 1 receptor expression in the intracardiac ganglia. Under voltage-clamp conditions, histamine evoked an inward current that was potentiated by extracellular Ca 2+ removal and attenuated by extracellular Na + replacement with N-methyl-D-glucamine. This implicated the involvement of non-selective cation channels, which given the link between H 1 receptors and G q/11 -protein-phospholipase C signalling, were suspected to be transient receptor potential canonical (TRPC) channels. This was confirmed by the marked inhibition of the inward current through the pharmacological disruption of either G q/11 signalling or intracellular Ca 2+ release and by the application of the TRPC blockers Pyr3, Gd 3+ and ML204. Consistently, RT-PCR analysis revealed the expression of several TRPC subtypes in the intracardiac ganglia. Whilst histamine was also separately found to inhibit the M-current, the histamine-induced depolarization was only significantly inhibited by the TRPC blockers Gd 3+ and ML204, and not by the M-current blocker XE991. These results suggest that TRPC channels serve as the predominant mediator of neuronal excitation by histamine., (Copyright © 2020 Elsevier B.V. All rights reserved.)

Background: Microglia, the principal sentinel immune cells of the central nervous system (CNS), play an extensively vital role in neuroinflammation and perioperative neurocognitive disorders (PND). Histamine, a potent mediator of inflammation, can both promote and prevent microglia-related neuroinflammation by activating different histamine receptors. Rat microglia express four histamine receptors (H1R, H2R, H3R, and H4R), among which the histamine 1 and 4 receptors can promote microglia activation, whereas the role and cellular mechanism of the histamine 2 and 3 receptors have not been elucidated. Therefore, we evaluated the effects and potential cellular mechanisms of histamine 2/3 receptors in microglia-mediated inflammation and PND., Methods: This study investigated the role of histamine 2/3 receptors in microglia-induced inflammation and PND both in vivo and in vitro. In the in vivo experiments, rats were injected with histamine 2/3 receptor agonists in the right lateral ventricle and were then subjected to exploratory laparotomy. In the in vitro experiments, primary microglia were pretreated with histamine 2/3 receptor agonists before stimulation with lipopolysaccharide (LPS). Cognitive function, microglia activation, proinflammatory cytokine production, NF-κb expression, M1/M2 phenotypes, cell migration, and Toll-like receptor-4 (TLR4) expression were assessed., Results: In our study, the histamine 2/3 receptor agonists inhibited exploratory laparotomy- or LPS-induced cognitive decline, microglia activation, proinflammatory cytokine production, NF-κb expression, M1/M2 phenotype transformation, cell migration, and TLR4 expression through the PI3K/AKT/FoxO1 pathway., Conclusion: Based on our findings, we conclude that histamine 2/3 receptors ameliorate PND by inhibiting microglia activation through the PI3K/AKT/FoxO1 pathway. Our results highlight histamine 2/3 receptors as potential therapeutic targets to treat neurological conditions associated with PND.

Chlamydia trachomatis infection is a primary cause of reproductive tract diseases including infertility. Previous studies showed that this infection alters physiological activities in mouse oviducts. Whether this occurs in the uterus and cervix has never been investigated. This study characterized the physiological activities of the uterine horn and the cervix in a Chlamydia muridarum ( Cmu )-infected mouse model at three infection time points of 7, 14, and 21 days postinfection (dpi). Cmu infection significantly decreased contractile force of spontaneous contraction in the cervix (7 and 14 dpi; P < 0.001 and P < 0.05, respectively), but this effect was not observed in the uterine horn. The responses of the uterine horn and cervix to oxytocin were significantly altered by Cmu infection at 7 dpi ( P < 0.0001), but such responses were attenuated at 14 and 21 dpi. Cmu infection increased contractile force to prostaglandin (PGF 2α ) by 53-83% in the uterine horn. This corresponded with the increased messenger ribonucleic acid (mRNA) expression of Ptgfr that encodes for its receptor. However, Cmu infection did not affect contractions of the uterine horn and cervix to PGE 2 and histamine. The mRNA expression of Otr and Ptger4 was inversely correlated with the mRNA expression of Il1b , Il6 in the uterine horn of Cmu -inoculated mice ( P < 0.01 to P < 0.001), suggesting that the changes in the Otr and Ptger4 mRNA expression might be linked to the changes in inflammatory cytokines. Lastly, this study also showed a novel physiological finding of the differential response to PGE 2 in mouse uterine horn and cervix.