Central histaminergic system and cognition

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

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

Dietary Seleno-l-Methionine Causes Alterations in Neurotransmitters, Ultrastructure of the Brain, and Behaviors in Zebrafish (Danio rerio)

Elevated concentrations of dietary selenium (Se) cause abnormalities and extirpation of fish inhabiting in Se-contaminated environments. However, its effect on fish behavior and the underlying mechanisms remain largely unknown. In this study, two-month-old zebrafish (Danio rerio) was fed seleno-l-methionine (Se-Met) at environmentally relevant concentrations (i.e., control (2.61), low (5.43), medium (12.16), and high (34.61) μg Se/g dry weight (dw), respectively, corresponding to the C, L, M, and H treatments) for 60 days. Targeted metabolomics, histopathological, and targeted transcriptional endpoints were compared to behavioral metrics to evaluate the effects of dietary exposure to Se-Met . The results showed that the levels of total Se and malondialdehyde in fish brains were increased in a dose-dependent pattern. Meanwhile, mitochondrial damages and decreased activities of the mitochondria respiratory chain complexes were observed in the neurons at the M and H treatments. In addition, dietary Se-Met affected neurotransmitters, metabolites, and transcripts of the genes associated with the dopamine, serotonin, gamma-aminobutyric acid, acetylcholine, and histamine signaling pathways in zebrafish brains at the H treatments. The total swimming distance and duration in the Novel Arm were lowered in fish from the H treatment. This study has demonstrated that dietary Se-Met affects the ultrastructure of the zebrafish brain, neurotransmitters, and associated fish behaviors and may help enhance adverse outcome pathways for neurotransmitter-behavior key events in zebrafish.

Histamine triggers microglial responses indirectly via astrocytes and purinergic signaling

Histamine is a monoaminergic neurotransmitter which is released within the entire brain from ascending axons originating in the tuberomammillary nucleus in a sleep state-dependent fashion. Besides the modulation of neuronal firing patterns, brain histamine levels are also thought to modulate functions of glial cells. Microglia are the innate immune cells and professional phagocytes of the central nervous system, and histamine was previously shown to have multiple effects on microglial functions in health and disease. Isolated microglia respond only to agonists of the Hrh2 subtype of histamine receptors (Hrh), and the expression of that isoform is confirmed by a metadata analysis of microglia transcriptomes. When we studied the effect of the histamine receptor isoforms in cortical and thalamic microglia by in situ live cell Ca²⁺ imaging using a novel, microglia-specific indicator mouse line, microglial cells respond to external histamine application mainly in a Hrh1-, and to a lower extent also in a Hrh2-dependent manner. The Hrh1 response was sensitive to blockers of purinergic P2ry12 receptors, and since Hrh1 expression was predominantly found in astrocytes, we suggest that the Hrh1 response in microglia is mediated by astrocyte ATP release and activation of P2ry12 receptors in microglia. Histamine also stimulates microglial phagocytic activity via Hrh1- and P2ry12-mediated signaling. Taken together, we provide evidence that histamine acts indirectly on microglial Ca²⁺ levels and phagocytic activity via astrocyte histamine receptor-controlled purinergic signaling.

Gut Biofactory-Neurocompetent Metabolites within the Gastrointestinal Tract. A Scoping Review

The gut microbiota have gained much scientific attention recently. Apart from unravelling the taxonomic data, we should understand how the altered microbiota structure corresponds to functions of this complex ecosystem. The metabolites of intestinal microorganisms, especially bacteria, exert pleiotropic effects on the human organism and contribute to the host systemic balance. These molecules play key roles in regulating immune and metabolic processes. A subset of them affect the gut brain axis signaling and balance the mental wellbeing. Neurotransmitters, short chain fatty acids, tryptophan catabolites, bile acids and phosphatidylcholine, choline, serotonin, and L-carnitine metabolites possess high neuroactive potential. A scoping literature search in PubMed/Embase was conducted up until 20 June 2020, using three major search terms "microbiota metabolites" AND "gut brain axis" AND "mental health". This review aimed to enhance our knowledge regarding the gut microbiota functional capacity, and support current and future attempts to create new compounds for future clinical interventions.

Neuronal histamine and the memory of emotionally salient events

In this review, we describe the experimental paradigms used in preclinical studies to unravel the histaminergic brain circuits that modulate the formation and retrieval of memories associated with aversive events. Emotionally arousing events, especially bad ones, are remembered more accurately, clearly and for longer periods of time than neutral ones. Maladaptive elaborations of these memories may eventually constitute the basis of psychiatric disorders such as generalized anxiety, obsessive-compulsive disorders and post-traumatic stress disorder. A better understanding of the role of the histaminergic system in learning and memory has not only a theoretical significance but also a translational value. Ligands of histamine receptors are among the most used drugs worldwide; hence, understanding the impact of these compounds on learning and memory may help improve their pharmacological profile and unravel unexplored therapeutic applications. LINKED ARTICLES: This article is part of a themed section on New Uses for 21st Century. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v177.3/issuetoc.

Histamine H3 receptor antagonists/inverse agonists: Where do they go?

Since the discovery of the histamine H₃ receptor in 1983, tremendous advances in the pharmacological aspects of H₃ receptor antagonists/inverse agonists have been accomplished in preclinical studies. At present, there are several drug candidates that reached clinical trial studies for various indications. However, entrance of these candidates to the pharmaceutical market is not free from challenges, and a variety of difficulties is engaged with their developmental process. In this review, the potential role of H₃ receptors in the pathophysiology of various central nervous system, metabolic and allergic diseases is discussed. Thereafter, the current status for H₃ receptor antagonists/inverse agonists in ongoing clinical trial studies is reviewed and obstacles in developing these agents are emphasized.

Hybrid approach to structure modeling of the histamine H3 receptor: Multi-level assessment as a tool for model verification

The crucial role of G-protein coupled receptors and the significant achievements associated with a better understanding of the spatial structure of known receptors in this family encouraged us to undertake a study on the histamine H3 receptor, whose crystal structure is still unresolved. The latest literature data and availability of different software enabled us to build homology models of higher accuracy than previously published ones. The new models are expected to be closer to crystal structures; and therefore, they are much more helpful in the design of potential ligands. In this article, we describe the generation of homology models with the use of diverse tools and a hybrid assessment. Our study incorporates a hybrid assessment connecting knowledge-based scoring algorithms with a two-step ligand-based docking procedure. Knowledge-based scoring employs probability theory for global energy minimum determination based on information about native amino acid conformation from a dataset of experimentally determined protein structures. For a two-step docking procedure two programs were applied: GOLD was used in the first step and Glide in the second. Hybrid approaches offer advantages by combining various theoretical methods in one modeling algorithm. The biggest advantage of hybrid methods is their intrinsic ability to self-update and self-refine when additional structural data are acquired. Moreover, the diversity of computational methods and structural data used in hybrid approaches for structure prediction limit inaccuracies resulting from theoretical approximations or fuzziness of experimental data. The results of docking to the new H3 receptor model allowed us to analyze ligand-receptor interactions for reference compounds.

Drosophila Vision Depends on Carcinine Uptake by an Organic Cation Transporter

Recycling of neurotransmitters is essential for sustained neuronal signaling, yet recycling pathways for various transmitters, including histamine, remain poorly understood. In the first visual ganglion (lamina) of Drosophila, photoreceptor-released histamine is taken up into perisynaptic glia, converted to carcinine, and delivered back to the photoreceptor for histamine regeneration. Here, we identify an organic cation transporter, CarT (carcinine transporter), that transports carcinine into photoreceptors during histamine recycling. CarT mediated in vitro uptake of carcinine. Deletion of the CarT gene caused an accumulation of carcinine in laminar glia accompanied by a reduction in histamine, resulting in abolished photoreceptor signal transmission and blindness in behavioral assays. These defects were rescued by expression of CarT cDNA in photoreceptors, and they were reproduced by photoreceptor-specific CarT knockdown. Our findings suggest a common role for the conserved family of CarT-like transporters in maintaining histamine homeostasis in both mammalian and fly brains.

Histaminergic modulation of cholinergic release from the nucleus basalis magnocellularis into insular cortex during taste aversive memory formation

The ability of acetylcholine (ACh) to alter specific functional properties of the cortex endows the cholinergic system with an important modulatory role in memory formation. For example, an increase in ACh release occurs during novel stimulus processing, indicating that ACh activity is critical during early stages of memory processing. During novel taste presentation, there is an increase in ACh release in the insular cortex (IC), a major structure for taste memory recognition. There is extensive evidence implicating the cholinergic efferents of the nucleus basalis magnocellularis (NBM) in cortical activity changes during learning processes, and new evidence suggests that the histaminergic system may interact with the cholinergic system in important ways. However, there is little information as to whether changes in cholinergic activity in the IC are modulated during taste memory formation. Therefore, in the present study, we evaluated the influence of two histamine receptor subtypes, H1 in the NBM and H3 in the IC, on ACh release in the IC during conditioned taste aversion (CTA). Injection of the H3 receptor agonist R-α-methylhistamine (RAMH) into the IC or of the H1 receptor antagonist pyrilamine into the NBM during CTA training impaired subsequent CTA memory, and simultaneously resulted in a reduction of ACh release in the IC. This study demonstrated that basal and cortical cholinergic pathways are finely tuned by histaminergic activity during CTA, since dual actions of histamine receptor subtypes on ACh modulation release each have a significant impact during taste memory formation.

Preclinical evaluation of non-imidazole histamine H3 receptor antagonists in comparison to atypical antipsychotics for the treatment of cognitive deficits associated with schizophrenia

Cognitive deficits associated with schizophrenia (CDS) are implicated as a core symptom cluster of the disease and are associated with poor daily life functioning. Unfortunately, current antipsychotic agents provide little alleviation of CDS, representing a critical unmet therapeutic need. Here we investigated the effects of ABT-239 and A-431404, non-imidazole histamine H(3) receptor (H(3)R) antagonists, in animal models with relevance to CDS. As N-methyl-d-aspartate receptor hypofunction is considered an important factor in the pathogenesis of schizophrenia, acute administration of ketamine or MK-801 was used to induce cognitive impairments. The assays employed in the current studies were spontaneous alternation in cross-maze, used as an indication of working memory, and inhibitory avoidance (IA), used to assess long-term memory retention. Risperidone and olanzapine were also tested to directly compare the effects of H(3)R antagonists to two widely used antipsychotics. ABT-239 and A-431404, but not risperidone and olanzapine, attenuated ketamine-induced deficits on spontaneous alternation in cross-maze, while none of these compounds affected alternation performance on their own. ABT-239 and A-431404 also attenuated MK-801-induced impairments in IA; no effects were observed when given alone. Risperidone and olanzapine, however, failed to attenuate MK-801-induced deficits in IA and produced dose-dependent impairments when given alone. ABT-239 was also investigated in methylazoxymethanol acetate (MAM) treated rats, a neurodevelopmental model for schizophrenia. Chronic, but not acute, treatment with ABT-239 significantly improved spontaneous alternation impairments in MAM rats tested in cross-maze. In summary, these results suggest H(3)R antagonists may have the potential to ameliorate CDS.

Histaminergic modulation of nonspecific plasticity of the auditory system and differential gating

In the auditory system of the big brown bat (Eptesicus fuscus), paired conditioned tonal (CS) and unconditioned leg stimuli (US) for auditory fear conditioning elicit tone-specific plasticity represented by best-frequency (BF) shifts that are augmented by acetylcholine, whereas unpaired CS and US for pseudoconditioning elicit a small BF shift and prominent nonspecific plasticity at the same time. The latter represents the nonspecific augmentations of auditory responses accompanied by the broadening of frequency tuning and decrease in threshold. It is unknown which neuromodulators are important in evoking the nonspecific plasticity. We found that histamine (HA) and an HA3 receptor (HA3R) agonist (α-methyl-HA) decreased, but an HA3R antagonist (thioperamide) increased, cortical auditory responses; that the HA3R agonist applied to the primary auditory cortex before pseudoconditioning abolished the nonspecific augmentation in the cortex without affecting the small cortical BF shift; and that antagonists of acetylcholine, norepinephrine, dopamine, and serotonin receptors did not abolish the nonspecific augmentation elicited by pseudoconditioning. The histaminergic system plays an important role in eliciting the arousal and defensive behavior, possibly through nonspecific augmentation. Thus HA modulates the nonspecific augmentation, whereas acetylcholine amplifies the BF shifts. These two neuromodulators may mediate differential gating of cortical plasticity.

SAR110894, a potent histamine H₃-receptor antagonist, displays procognitive effects in rodents

SAR110894 is a novel histamine H₃-R ligand, displaying high and selective affinity for human, rat or mouse H₃-Rs. SAR110894 is a potent H₃-R antagonist at native receptors, reversing R-α-methylhistamine-induced inhibition of electrical field stimulation contraction in the guinea-pig ileum. Additionally, SAR110894 inhibited constitutive GTPγS binding at human H₃-Rs demonstrating inverse agonist properties. In behavioral models addressing certain aspects of cognitive impairment associated with schizophrenia (CIAS) and attention deficit/hyperactivity disorder (ADHD), SAR110894 improved memory performances in several variants of the object recognition task in mice (0.3-3 mg/kg, p.o.) or rats (0.3-1 mg/kg, p.o.). Moreover, SAR110894 (1 mg/kg, p.o.) reversed a deficit in working memory in the Y-maze test, following an acute low dose of phencyclidine (PCP) (0.5 mg/kg, i.p.) in mice sensitized by repeated treatment with a high dose of PCP (10 mg/kg, i.p.). In the latent inhibition (LI) model, SAR110894 potentiated LI in saline-treated rats (1 and 3 mg/kg, i.p.) and reversed abnormally persistent LI induced by neonatal nitric oxide synthase (NOS) inhibition in rodents (0.3-3 mg/kg, i.p.). In a social novelty discrimination task in rats, SAR110894 attenuated selective attention deficit induced by neonatal PCP treatment (3 and 10 mg/kg, p.o.) or a parametric modification of the procedure (3 and 10 mg/kg, p.o.). SAR110894 showed efficacy in several animal models related to the cognitive deficits in Alzheimer's disease (AD). It prevented the occurrence of episodic memory deficit induced by scopolamine in rats (0.01-10 mg/kg, p.o.) or by the central infusion of the toxic amyloid fragment β₂₅₋₃₅ in the object recognition test in mice (1 and 3 mg/kg, p.o.). Altogether, these findings suggest that SAR110894 may be of therapeutic interest for the treatment of the cognitive symptoms of AD, schizophrenia and certain aspects of ADHD.

Additive effects of a cholinesterase inhibitor and a histamine inverse agonist on scopolamine deficits in humans

RATIONALE

Enhancement of histaminergic neurotransmission or histaminergic plus cholinergic neurotransmission may represent novel strategies for improving cognition in Alzheimer's disease.

OBJECTIVE

To evaluate the effects of a novel histamine H3 receptor inverse agonist (MK-3134), an acetylcholinesterase inhibitor (donepezil), and their combination in attenuating the cognitive impairment associated with scopolamine.

METHODS

Thirty-one subjects were randomized, and 28 completed this double-blind, placebo-controlled, five-period crossover study. Cognition was assessed using the Groton Maze Learning Task (GMLT) as the primary outcome measure. The two primary hypotheses were that donepezil 10 mg and MK-3134 25 mg, respectively, would attenuate scopolamine (0.5 mg)-induced impairment as measured by the GMLT over the first 12 h after scopolamine administration (AUC(1-12)  (h)). A secondary hypothesis was that the combination of donepezil and MK-3134 would attenuate scopolamine-induced cognitive impairment to a greater extent than either agent alone as measured by the GMLT AUC(1-12 h).

RESULTS

The primary and secondary hypotheses were not met. Upon examining the time course of the scopolamine effects (an exploratory objective), peak effects were generally observed around 2 h after scopolamine administration. Administration of MK-3134 or donepezil improved performance on the GMLT at the 2-h time point, rather than AUC(1-12 h), compared with scopolamine alone. Moreover, it appeared that the combination of MK-3134 and donepezil blunted the scopolamine effect to a greater extent than either drug alone.

CONCLUSIONS

Exploratory analyses provide evidence for cognitive improvement through inverse agonism of the H3 histamine receptor and for cooperation between human cholinergic and histaminergic neurotransmitter systems. (ClinicalTrials.gov trial registration number: NCT01181310).

Memory in humans is unaffected by central H1-antagonism, while objectively and subjectively measured sedation is increased

Animal literature suggests an important role for histamine in memory. In humans, this hypothesis has been scarcely tested and results from studies that have addressed this are conflicting. Second, impaired memory performance may be secondary to sedation. This study aimed to determine whether a centrally active antihistamine impairs memory performance and to dissociate such effects from sedation. Eighteen healthy volunteers received single oral doses of dexchlorpheniramine 4 mg, lorazepam 1mg and placebo in a 3-way, double blind, crossover designed study. The active control lorazepam impaired episodic- and working memory performance and increased sedation, while dexchlorpheniramine only increased sedation.

Histamine H3 receptor activation decreases kainate-induced hippocampal gamma oscillations in vitro by action potential desynchronization in pyramidal neurons

The study of rhythmic electrical activity in slice preparations has generated important insights into neural network function. While the synaptic mechanisms involved in the generation of in vitro network oscillations have been studied widely, little is known about the modulatory influence exerted on rhythmic activity in neuronal networks by neuropeptides and biogenic amines. Gamma oscillations play an important role in cognitive processes and are altered or disrupted in disorders such as Alzheimer's disease (AD) and schizophrenia. Given the importance of gamma oscillations for learning, memory and cognition processes as well as the recent interest in histamine H(3) receptors in the development of pro-cognitive drugs to treat disorders such as AD and schizophrenia, it is relevant to study the impact of histaminergic mechanisms on network gamma oscillations. Here we show for the first time a modulation of gamma oscillation by histaminergic mechanisms. Selective activation of the H(3) receptor by R-alpha-methylhistamine significantly reduces the power of kainate-induced gamma oscillations, but not carbachol-induced gamma oscillations, in the rat hippocampal slice preparation without affecting oscillation frequency. This effect is neither caused by a decrease in excitatory or inhibitory postsynaptic currents, nor a decrease in cellular excitability. Instead, we find that the decrease in oscillation power following H(3) receptor activation results from a desynchronization of pyramidal neuron action potential firing with regard to the local field potential oscillation cycle. Our data provide a possible mechanism of action for histamine in regulating gamma oscillations in the hippocampal network.

Blockade of nucleus basalis magnocellularis or activation of insular cortex histamine receptors disrupts formation but not retrieval of aversive taste memory

Recent research, using several experimental models, demonstrated that the histaminergic system is clearly involved in memory formation. This evidence suggested that during different associative learning tasks, histamine receptor subtypes have opposite functions, related to the regulation of cortical cholinergic activity. Given that cortical cholinergic activity and nucleus basalis magnocellularis (NBM) integrity are needed during taste memory formation, the aim of this study was to determine the role of histamine receptors during conditioned taste aversion (CTA). We evaluated the effects of bilateral infusions of 0.5 microl of pyrilamine (100 mM), an H(1) receptor antagonist, into the NBM, or of R-alpha-methylhistamine (RAMH) (10 mM), an H(3) receptor agonist, into the insular cortex of male Sprague-Dawley rats 20 min before acquisition and/or retrieval of conditioned taste aversion. The results showed that blockade of H(1) receptors in NBM or activation of H(3) receptors in the insular cortex impairs formation but not retrieval of aversive taste memory. These results demonstrated differential roles for histamine receptors in two important areas for taste memory formation and suggest that these effects could be related with the cortical cholinergic activity modulation during CTA acquisition.

Histamine H1 receptor blockade predominantly impairs sensory processes in human sensorimotor performance

BACKGROUND AND PURPOSE

Centrally active antihistamines impair cognitive performance, particularly sensorimotor performance. The aim of the present study was to further elucidate the scarcely studied subprocesses involved in sensorimotor performance, which may be affected by H1 receptor blockade. Better knowledge about the cognitive deficits associated with histamine dysfunction can contribute to better treatment of clinical disorders in which histamine hypofunction may be a contributing factor, such as in schizophrenia.

EXPERIMENTAL APPROACH

Interactions of dexchlorpheniramine with specific task manipulations in a choice reaction time task were studied. Task demands were increased at the level of sensory subprocesses by decreasing stimulus quality, and at the level of motor subprocesses by increasing response complexity. A total of 18 healthy volunteers (9 female) aged between 18 and 45 years participated in a three-way, double-blind, crossover design. Treatments were single oral doses of 4 mg dexchlorpheniramine, 1 mg lorazepam and placebo. Behavioural effects were assessed by measuring reaction times and effects on brain activity by event-related potentials.

KEY RESULTS

Dexchlorpheniramine significantly slowed reaction times, but did not significantly interact with task manipulations. However, it did significantly interact with stimulus quality, as measured by event-related potentials. Lorazepam slowed reaction times and interacted with perceptual manipulations, as shown by effects on reaction times.

CONCLUSIONS AND IMPLICATIONS

The results confirm that the histamine system is involved in sensory information processing and show that H1 blockade does not affect motoric information processing. Histamine hypofunction in clinical disorders may cause impaired sensory processing, which may be a drug target.

Classifying antipsychotic agents : need for new terminology

Converging data from multiple lines of research provide growing understanding of the pharmacological basis of the efficacy and tolerability of antipsychotic agents. This review highlights some of the drawbacks of the current practice of classifying antipsychotic agents into first- and second-generation agents, and argues that much of what is known about an antipsychotic agent in terms of its efficacy and tolerability can be predicted from its binding affinity at different receptors. This makes a case for a new system of classification that reflects the receptor binding affinity profiles of individual antipsychotic agents. In its quest to make a compelling case, the review provides detailed explanations for the pharmacological basis of antipsychotic efficacy, antipsychotic-induced weight gain and diabetes mellitus, cognitive effects and other adverse effects.

H3 receptor antagonists reverse delay-dependent deficits in novel object discrimination by enhancing retrieval

RATIONALE

Accumulated evidence suggests a role for histamine in cognition and the use of H3 receptor antagonists in the treatment of learning and memory disorders.

OBJECTIVES

The aim of the current study was to investigate the cognition enhancing properties of ciproxifan, an H3 receptor antagonist, after natural forgetting in normal adult rats.

MATERIALS AND METHODS

The novel object discrimination task, a recognition memory test based on spontaneous exploratory behaviour, was used. Briefly, rats exposed to two identical objects during an acquisition trial can discriminate between a novel object and a familiar one during a subsequent choice trial after a short delay but not after a 24-h inter-trial interval.

RESULTS

The scopolamine (0.5 mg/kg, i.p.)-induced impairment after a short delay was abolished by ciproxifan (p < 0.001). Natural forgetting was prevented by a single administration of ciproxifan (3 mg/kg) prior to the retention test (p < 0.001) but not when administered before or immediately after the acquisition trial (schedule effect p < 0.05), demonstrating a specific activity on memory retrieval. Pretreatment with either pyrilamine (10 mg/kg), an H1 antagonist, or zolantidine (10 mg/kg), an H2 antagonist, prevented the retrieval enhancement effect of ciproxifan (p < 0.05 and p < 0.001, respectively).

CONCLUSIONS

Histamine H3 receptor antagonists restore the performance of rats impaired by scopolamine and enhance recognition memory after acute administration before the retrieval phase via a mechanism dependent on H1 and H2 receptor activation.

The role of histamine on cognition

Histamine was intensively studied at the beginning of the 20th century because of its important role in allergic and inflammation processes. In those days it was very difficult that researchers could envisage another impacting function for the imidazolamine in the living systems. Once the imidazolamine was found located in neuron compartment in the brain, increasing evidence supported many regulatory functions including its possible role in memory and learning. The specific participation of histamine in cognitive functions followed a slow and unclear pathway because the many different experimental learning models, pharmacologic approaches, systemic and localized applications of the histamine active compounds into the brain used by researchers showed facilitating or inhibitory effects on learning, generating an active issue that has extended up to present time. In this review, all these aspects are analyzed and discussed considering the many intracellular different mechanisms discovered for histamine, the specific histamine receptors and the compartmentalizing proprieties of the brain that might explain the apparent inconsistent effects of the imidazolamine in learning. In addition, a hypothetical physiologic role for histamine in memory is proposed under the standard theories of learning in experimental animals and humans.

Modafinil: a review of neurochemical actions and effects on cognition

Modafinil (2-[(Diphenylmethyl) sulfinyl] acetamide, Provigil) is an FDA-approved medication with wake-promoting properties. Pre-clinical studies of modafinil suggest a complex profile of neurochemical and behavioral effects, distinct from those of amphetamine. In addition, modafinil shows initial promise for a variety of off-label indications in psychiatry, including treatment-resistant depression, attention-deficit/hyperactivity disorder, and schizophrenia. Cognitive dysfunction may be a particularly important emerging treatment target for modafinil, across these and other neuropsychiatric disorders. We aimed to comprehensively review the empirical literature on neurochemical actions of modafinil, and effects on cognition in animal models, healthy adult humans, and clinical populations. We searched PubMed with the search term 'modafinil' and reviewed all English-language articles for neurochemical, neurophysiological, cognitive, or information-processing experimental measures. We additionally summarized the pharmacokinetic profile of modafinil and clinical efficacy in psychiatric patients. Modafinil exhibits robust effects on catecholamines, serotonin, glutamate, gamma amino-butyric acid, orexin, and histamine systems in the brain. Many of these effects may be secondary to catecholamine effects, with some selectivity for cortical over subcortical sites of action. In addition, modafinil (at well-tolerated doses) improves function in several cognitive domains, including working memory and episodic memory, and other processes dependent on prefrontal cortex and cognitive control. These effects are observed in rodents, healthy adults, and across several psychiatric disorders. Furthermore, modafinil appears to be well-tolerated, with a low rate of adverse events and a low liability to abuse. Modafinil has a number of neurochemical actions in the brain, which may be related to primary effects on catecholaminergic systems. These effects are in general advantageous for cognitive processes. Overall, modafinil is an excellent candidate agent for remediation of cognitive dysfunction in neuropsychiatric disorders.

The histamine H3 receptor: an attractive target for the treatment of cognitive disorders

The histamine H3 receptor, first described in 1983 as a histamine autoreceptor and later shown to also function as a heteroreceptor that regulates the release of other neurotransmitters, has been the focus of research by numerous laboratories as it represents an attractive drug target for a number of indications including cognition. The purpose of this review is to acquaint the reader with the current understanding of H3 receptor localization and function as a modulator of neurotransmitter release and its effects on cognitive processes, as well as to provide an update on selected H3 antagonists in various states of preclinical and clinical advancement. Blockade of centrally localized H3 receptors by selective H3 receptor antagonists has been shown to enhance the release of neurotransmitters such as histamine, ACh, dopamine and norepinephrine, among others, which play important roles in cognitive processes. The cognitive-enhancing effects of H3 antagonists across multiple cognitive domains in a wide number of preclinical cognition models also bolster confidence in this therapeutic approach for the treatment of attention deficit hyperactivity disorder, Alzheimer's disease and schizophrenia. However, although a number of clinical studies examining the efficacy of H3 receptor antagonists for a variety of cognitive disorders are currently underway, no clinical proof of concept for an H3 receptor antagonist has been reported to date. The discovery of effective H3 antagonists as therapeutic agents for the novel treatment of cognitive disorders will only be accomplished through continued research efforts that further our insights into the functions of the H3 receptor.

Atomoxetine increases histamine release and improves learning deficits in an animal model of attention-deficit hyperactivity disorder: the spontaneously hypertensive rat

Substantial development in the pharmacological treatment for attention-deficit hyperactivity disorder (ADHD) has been made recently including approval of new non-stimulant agents targeting noradrenergic, histaminergic and dopaminergic systems. Among such, atomoxetine has been widely used, although its mechanism of action is poorly understood. It is known that central nervous system histamine is closely associated with cognition and it was recently shown that both atomoxetine and methylphenidate enhance cortical histamine release in rats. To that end, the aim of our study was to investigate the effect of atomoxetine (2 mg/kg, intraperitoneally) on histamine release using the microdialysis technique in the spontaneously hypertensive rat (SHR), a suitable genetic model for ADHD. Our data confirmed that atomoxetine increases extracellular levels of histamine in the prefrontal cortex, a brain region that is implicated in the pathophysiology of ADHD. Given the tie between histamine neurotransmission and treatment of cognitive dysfunction, we also assessed the effects of atomoxetine on learning and memory as measured by the Morris water maze in SHR. The results indicated that atomoxetine significantly ameliorated performance in the Morris water maze, consistent with its histamine-enhancing profile. In conclusion, the current study provides further support for the notion that the therapeutic effect of atomoxetine could involve activation of histamine neurotransmission within the prefrontal cortex.

The histamine N-methyltransferase T105I polymorphism affects active site structure and dynamics

Histamine N-methyltransferase (HNMT) is the primary enzyme responsible for inactivating histamine in the mammalian brain. The human HNMT gene contains a common threonine-isoleucine polymorphism at residue 105, distal from the active site. The 105I variant has decreased activity and lower protein levels than the 105T protein. Crystal structures of both variants have been determined but reveal little regarding how the T105I polymorphism affects activity. We performed molecular dynamics simulations for both 105T and 105I at 37 degrees C to explore the structural and dynamic consequences of the polymorphism. The simulations indicate that replacing Thr with the larger Ile residue leads to greater burial of residue 105 and heightened intramolecular interactions between residue 105 and residues within helix alpha3 and strand beta3. This altered, tighter packing is translated to the active site, resulting in the reorientation of several cosubstrate-binding residues. The simulations also show that the hydrophobic histamine-binding domain in both proteins undergoes a large-scale breathing motion that exposes key catalytic residues and lowers the hydrophobicity of the substrate-binding site.

Carnosine protects against Abeta42-induced neurotoxicity in differentiated rat PC12 cells

(1) The present study was designed to investigate whether histamine is involved in the protective effect of carnosine on Abeta42-induced impairment in differentiated PC12 cells. (2) PC12 cells were exposed to Abeta42 (5 muM) for 24 h after carnosine (5 mM) applied for 18 h. Histamine receptor antagonists (diphenhydramine, zolantidine, thioperamide, clobenpropit) or histidine decarboxylase inhibitor (alpha-fluoromethylhistidine) were added 15 min before carnosine. Cell viability, glutamate release or cell surface expression of NMDA receptor was examined. (3) Abeta42 caused a concentration-dependent reduction of viability in PC12 cells and pretreatment with carnosine ameliorated this impairment. This amelioration was reversed by the H(3) receptor antagonists thioperamide and clobenpropit, but not by either the H(1) receptor antagonist diphenhydramine or the H(2) receptor antagonist zolantidine. Further, alpha-fluoromethylhistidine, an irreversible inhibitor of histidine decarboxylase, also had no effect. In the presence of Abeta42, carnosine significantly decreased glutamate release and carnosine increased the surface expression of NMDA receptor. (4) These results indicate that the mechanism by which carnosine attenuates Abeta42-induced neurotoxicity is independent of the carnosine-histidine-histamine pathway, but may act through regulation of glutamate release and NMDA receptor trafficking.

Histamine H3 receptor ligands break ground in a remarkable plethora of therapeutic areas

The neurotransmitter histamine exerts its action through four distinct histamine receptors. The histamine H(1) and H(2) receptor are well established drug targets, whereas the histamine H(4) receptor is undergoing rigorous characterisation at present. The histamine H(3) receptor (H(3)R) is a G(i/o)-protein coupled receptor and is mostly expressed in the CNS. A remarkably large and different array of therapeutic areas, in which ligands for the H(3)R may prove useful, has been identified and a massive research undertaking is underway to substantiate the high expectations for H(3)R ligands. At present, several ligands for the H(3)R are being evaluated in clinical studies. In this review, the many potential therapeutic areas for H(3)R antagonists, inverse agonists and agonists is discussed. Promising medicinal chemistry and toxicological developments, as well as the advancement of several H(3)R ligands into the clinic, will be highlighted. This review also describes the problems that have been overcome and the questions that remain in developing H(3)R-related drugs. Considering the tremendous efforts by industry, it can be expected that the first H(3)R drugs will reach the market soon.

Lurasidone (SM-13496), a novel atypical antipsychotic drug, reverses MK-801-induced impairment of learning and memory in the rat passive-avoidance test

Lurasidone (SM-13496) is a novel atypical antipsychotic with high affinities to dopamine D2, serotonin 5-HT7, 5-HT2A, 5-HT1A receptors and alpha2C adrenoceptor. In this study, the effects of lurasidone on the rat passive-avoidance response and its impairment by the N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 (dizocilpine) were evaluated and compared with those of other antipsychotics. The passive-avoidance response was examined by measuring the step-through latency, 1 day after the animals received foot-shock training. When given before the training session, lurasidone did not affect the passive-avoidance response at any dose tested (1-30 mg/kg, p.o.). All the other atypical antipsychotics examined (i.e., risperidone, olanzapine, quetiapine, clozapine and aripiprazole), however, significantly reduced the step-through latency at relatively high doses. A pre-training administration of lurasidone significantly and dose-dependently reversed the MK-801-induced impairment of the passive-avoidance response. At doses lower than those that affected the passive-avoidance response, risperidone, quetiapine, and clozapine partially reduced the MK-801-induced impairment, whereas haloperidol, olanzapine, and aripiprazole were inactive. In addition, the post-training administration of lurasidone was as effective in countering the MK-801 effect as the pre-training administration, suggesting that lurasidone worked, at least in part, by restoring the memory consolidation process disrupted by MK-801. These results suggest that lurasidone is superior to other antipsychotics in improving the MK-801-induced memory impairment and may be clinically useful for treating cognitive impairments in schizophrenia.

Selective cognitive dysfunction in mice lacking histamine H1 and H2 receptors

Previous pharmacological experiments provide conflicting findings that describe both facilitatory and inhibitory effects of neuronal histamine on learning and memory. Here, we examined learning and memory and synaptic plasticity in mice with a null mutation of gene coding histamine H1 or H2 receptor in order to clarify the role of these receptors in learning and memory processes. Learning and memory were evaluated by several behavioral tasks including object recognition, Barnes maze and fear conditioning. These behavioral tasks are highly dependent on the function of prefrontal cortex, hippocampus or amygdala. Object recognition and Barnes maze performance were significantly impaired in both H1 receptor gene knockout (H1KO) and H2 receptor gene knockout (H2KO) mice when compared to the respective wild-type (WT) mice. Conversely, both H1KO and H2KO mice showed better auditory and contextual freezing acquisition than their respective WT mice. Furthermore, we also examined long-term potentiation (LTP) in the CA1 area of hippocampus in H1KO and H2KO mice and their respective WT mice. LTP in the CA1 area of hippocampus was significantly reduced in both H1KO and H2KO mice when compared with their respective WT mice. In conclusion, our results demonstrate that both H1 and H2 receptors are involved in learning and memory processes for which the frontal cortex, amygdala and hippocampus interact.

Involvement of histamine receptors in the acquisition of inhibitory avoidance in Carassius auratus

This study investigated the involvement of H(1) and H(2) histaminegic receptors on the acquisition of a new task in Carassius auratus by using an inhibitory avoidance paradigm in which the animals had to learn to avoid an aversive stimulus. Before training, the fish received injections of H(2) antagonist zolantidine at a dose of 20 mg/kg, or H(1) antagonist chlorpheniramine at a dose of 4 or 16 mg/kg. Control animals were injected with distilled water. A facilitatory effect of chlorpheniramine was observed at the dose of 16 mg/kg. On the other hand, the administration of 20 mg/kg of zolantidine inhibited acquisition. Place preference conditioning was used to observe the aversive or reinforcing effects of the drugs, which could interfere with the inhibitory avoidance procedure; however, no effects were observed. Thus, it can be suggested that both receptors, H(1) and H(2), are involved in the acquisition of a new task in this species.

Evidence for tolerance following repeated dosing in rats with ciproxifan, but not with A-304121

Blockade of presynaptic histamine H(3) receptors with potent and selective ligands improves cognitive function in rodents and there is significant interest in developing such drugs for long-term symptomatic treatment of CNS disorders such as attention deficit hyperactivity disorder (ADHD). Unfortunately, little is known about the effects of repeated exposure to H(3) receptor antagonists/inverse agonists. We therefore investigated the effects of acute and repeated daily administration of two potent, brain penetrating H(3) receptor antagonists/inverse agonists, ciproxifan and A-304121, on rat body weight, food and water intake, core temperature and locomotor activity, as well as H(3) receptor density and gene expression levels. Methylphenidate, used clinically for the treatment of ADHD, was included as an additional comparator. Ciproxifan, an imidazole-based compound, decreased food intake over the first 10 days and locomotor activity acutely, but these effects were lost after further repeated administration. The ex vivo binding studies revealed increased H(3) receptor density in rats following repeated administration of ciproxifan for 10 or 15 days; however, H(3) receptor gene expression was not changed. In contrast, rats treated with the non-imidazole, A-304121, did not differ from controls on any measure during the observation period, while rats treated with methylphenidate exhibited hyperthermia and hyperactivity. The implications for potential long-term treatment with H(3) receptor antagonists in CNS disorders such as ADHD are discussed.

Enhanced antinociceptive effects of morphine in histamine H2 receptor gene knockout mice

We have previously shown that antinociceptive effects of morphine are enhanced in histamine H1 receptor gene knockout mice. In the present study, involvement of supraspinal histamine H2 receptor in antinociception by morphine was examined using histamine H2 receptor gene knockout (H2KO) mice and histamine H2 receptor antagonists. Antinociception was evaluated by assays for thermal (hot-plate, tail-flick and paw-withdrawal tests), mechanical (tail-pressure test) and chemical (formalin and capsaicin tests) stimuli. Thresholds for pain perception in H2KO mice were higher than wild-type mice. Antinociceptive effects of intracerebroventricularly administered morphine were enhanced in the H2KO mice compared to wild-type mice. Intracerebroventricular co-administration of morphine and cimetidine produced significant antinociceptive effects in the wild-type mice when compared to morphine or cimetidine alone. Furthermore, zolantidine, a selective and hydrophobic H2 receptor antagonist, enhanced the effects of morphine in all nociceptive assays examined. These results suggest that histamine exerts inhibitory effects on morphine-induced antinociception through H2 receptors at the supraspinal level. Our present and previous studies suggest that H1 and H2 receptors cooperatively function to modulate pain perception in the central nervous system.

Histamine protects against NMDA-induced necrosis in cultured cortical neurons through H receptor/cyclic AMP/protein kinase A and H receptor/GABA release pathways

Using histamine and the H3 receptor antagonist thioperamide, the roles of histamine receptors in NMDA-induced necrosis were investigated in rat cultured cortical neurons. Within 3 h of intense NMDA insult, most neurons died by necrosis. Histamine reversed the neurotoxicity in a concentration-dependent manner and showed peak protection at a concentration of 10(-7) m. This protection was antagonized by the H2 receptor antagonists cimetidine and zolantidine but not by the H1 receptor antagonists pyrilamine and diphenhydramine. In addition, the selective H2 receptor agonist amthamine mimicked the protection by histamine. This action was prevented by cimetidine but not by pyrilamine. 8-Bromo-cAMP also mimicked the effect of histamine. In contrast, both the adenylyl cyclase inhibitor 9-(tetrahydro-2-furanyl)-9H-purine-6-amine and the cAMP-dependent protein kinase inhibitor N-[2-(p-bromocinnamylamino) ethyl]-5-isoquinolinesulfonamide reversed the protection by histamine. Thioperamide also attenuated NMDA-induced excitotoxicity, which was reversed by the H3 receptor agonist (R)-alpha-methylhistamine but not by pyrilamine and cimetidine. In addition, the protection by thioperamide was inhibited by the GABA(A) receptor antagonists picrotoxin and bicuculline. Further study demonstrated that the protection by thioperamide was due to increased GABA release in NMDA-stimulated samples. These results indicate that not only the H2 receptor/cAMP/cAMP-dependent protein kinase pathway but also the H3 receptor/GABA release pathway can attenuate NMDA-induced neurotoxicity.

Effect of endogenous histamine in the ventral hippocampus on fear memory deficits induced by scopolamine as evaluated by step-through avoidance response in rats

In the present study, the effects of endogenous histamine in the ventral hippocampus on fear memory deficits induced by scopolamine were investigated as evaluated by step-through avoidance response in adult male rats. Bilateral ventral hippocampal injection of scopolamine (i.h., 2, 5 microg/site) significantly produced depressant effects on the active avoidance response in a dose-dependent manner. Histamine H(3)-antagonist clobenpropit (5, 10 microg/site) significantly ameliorated the fear memory deficits induced by scopolamine in a dose-dependent manner. Its procognitive effect was completely antagonized by immepip (10 microg/site), a selective histamine H(3)-agonist. Both histamine H(1)-antagonist pyrilamine and H(2)-antagonist cimetidine, also inhibited the procognitive effects of clobenpropit. Additionally, the procognitive effects of clobenpropit on the fear memory deficits induced by scopolamine were significantly potentiated by intraperitoneal (i.p.) injection of histidine, a precursor of histamine, but markedly reversed by i.h. injection of alpha-fluoromethylhistidine (FMH, 10 microg/site), a selective and potent histidine decarboxylase inhibitor. It is concluded that the increased endogenous histamine release in the ventral hippocampus ameliorates the scopolamine-induced fear memory deficits, and its action is mainly mediated by histamine presynaptic H(3)-receptors and postsynaptic H(1)- and H(2)-receptors.

Age-dependent measures of anxiety and cognition in male histidine decarboxylase knockout (Hdc−/−) mice

Histidine decarboxylase deficient (Hdc(-/-)) and wild-type male mice on the C57Bl6/J background were used to determine the role of histamine in brain function. 3-5 (Y) and 12-14 (MA) month-old Hdc(-/-) mice showed hypoactivity and increased measures of anxiety in the open field, light-dark, elevated plus-maze, and elevated zero maze tests. Y Hdc(-/-) mice showed superior performance in the hidden sessions of the water maze and passive avoidance memory retention. In contrast, Y Hdc(-/-) mice were impaired in novel location recognition, spent less time searching in the target quadrant and more time searching in the outer zone of the water maze during the probe trials. These behaviors are likely due to increased measures of anxiety and are not found in MA Hdc(-/-) mice. These data support a role for histamine in anxiety and cognition and underline the importance of considering age and potential effects on measures of anxiety in the interpretation of the role of histaminergic neurotransmission in cognitive function.

Histamine ameliorates spatial memory deficits induced by MK-801 infusion into ventral hippocampus as evaluated by radial maze task in rats

AIM

To investigate the role of histamine in memory deficits induced by MK-801 infusion into the ventral hippocampus in rats.

METHODS

An 8-arm radial maze (4 arms baited) was used to assess spatial memory.

RESULTS

Bilateral ventral intrahippocampal (ih) infusion of MK-801 (0.3 microg/site), an N-methyl-D-aspartate (NMDA) antagonist, impaired the retrieval process in both working memory and reference memory. Intrahippocampal injection of histamine (25 or 50 ng/site) or intraperitoneal (ip) injection of histidine (25, 50 or 100 mg/kg) markedly ameliorated the spatial memory deficits induced by MK-801. Both the histamine H1 antagonist pyrilamine (0.5 or 1.0 microg/site, ih) and the H2 antagonist cimetidine (2.5 microg/site, ih) abolished the ameliorating effect of histidine (100 mg/kg, ip) on reference memory deficits, but not that on working memory deficits induced by MK-801.

CONCLUSION

The results indicate that histamine in the ventral hippocampus can ameliorate MK-801-induced spatial memory deficits, and that histamine's effect on reference memory is mediated by postsynaptic histamine H1 and H2 receptors.

The histamine H3 receptor: from gene cloning to H3 receptor drugs

Since the cloning of the histamine H(3) receptor cDNA in 1999 by Lovenberg and co-workers, this histamine receptor has gained the interest of many pharmaceutical companies as a potential drug target for the treatment of various important disorders, including obesity, attention-deficit hyperactivity disorder, Alzheimer's disease, schizophrenia, as well as for myocardial ischaemia, migraine and inflammatory diseases. Here, we discuss relevant information on this target protein and describe the development of various H(3) receptor agonists and antagonists, and their effects in preclinical animal models.

The histamine H3 receptor as a novel therapeutic target for cognitive and sleep disorders

Histamine H3 receptor pharmacology, functions and biochemistry are far from being fully understood; however, progress is being made. Activation of this Gi/GO-protein-coupled receptor affects cognition, the sleep-wake cycle, obesity and epilepsy, which are physiological and pathological conditions that are the main focus of research into the therapeutic potential of selective H3 receptor ligands. This heterogeneity of targets can be reconciled partially by the fact that the histamine system constitutes one of the most important brain-activating systems and that H3 receptors regulate the activity of histamine and other neurotransmitter systems. Furthermore, the H3 receptor shows functional constitutive activity, polymorphisms in humans and rodents with a differential distribution of splice variants in the CNS, and potential coupling to different intracellular signal transduction mechanisms. In light of the genetic, pharmacological and functional complexity of the H3 receptor, the importance of the histamine system as a therapeutic target to control the sleep-wake cycle and cognitive disorders will be discussed.

Histamine innervation and activation of septohippocampal GABAergic neurones: involvement of local ACh release

Recent studies indicate that the histaminergic system, which is critical for wakefulness, also influences learning and memory by interacting with cholinergic systems in the brain. Histamine-containing neurones of the tuberomammillary nucleus densely innervate the cholinergic and GABAergic nucleus of the medial septum/diagonal band of Broca (MSDB) which projects to the hippocampus and sustains hippocampal theta rhythm and associated learning and memory functions. Here we demonstrate that histamine, acting via H(1) and/or H(2) receptor subtypes, utilizes direct and indirect mechanisms to excite septohippocampal GABA-type neurones in a reversible, reproducible and concentration-dependent manner. The indirect mechanism involves local ACh release, is potentiated by acetylcholinesterase inhibitors and blocked by atropine methylbromide and 4-DAMP mustard, an M(3) muscarinic receptor selective antagonist. This indirect effect, presumably, results from a direct histamine-induced activation of septohippocampal cholinergic neurones and a subsequent indirect activation of the septohippocampal GABAergic neurones. In double-immunolabelling studies, histamine fibres were found in the vicinity of both septohippocampal cholinergic and GABAergic cell types. These findings have significance for Alzheimer's disease and other neurodegenerative disorders involving a loss of septohippocampal cholinergic neurones as such a loss would also obtund histamine effects on septohippocampal cholinergic and GABAergic functions and further compromise hippocampal arousal and associated cognitive functions.

Enhancement of prepulse inhibition of startle in mice by the H3 receptor antagonists thioperamide and ciproxifan

Histamine H3 receptor antagonists/inverse agonists have been proposed as potential therapeutic agents for the treatment of a number of neurological disorders ranging from attention deficit hyperactivity disorder and Alzheimer's disease to narcolepsy and schizophrenia. With respect to the latter, schizophrenic patients typically exhibit impaired prepulse inhibition (PPI) of startle, a reflex that can be modeled in many animal species. Certain strains of mice naturally display poor PPI and it was recently suggested that these mice might offer a new way to screen for novel antipsychotic compounds. To examine whether H3 receptor antagonists might enhance PPI in mice with naturally occurring deficits, DBA/2 and C57BL/6 were tested in a startle paradigm with three prepulse intensities: 5, 10 and 15 dB above background. Both thioperamide and ciproxifan enhanced PPI in the DBA/2 strain; thioperamide also showed a trend towards enhancing PPI in C57BL/6. Risperidone, an atypical antipsychotic, enhanced PPI in both the DBA/2 and the C57BL/6 strain. These data confirm previous reports describing a natural deficit in PPI in some mouse strains that is amenable to enhancement with known antipsychotics. Further, these data suggest that H3 receptor antagonists/inverse agonists have anti-psychotic potential for disorders such as schizophrenia.